DOC: Handle exceptions when computing contributors.
We're failing on master right now https://travis-ci.org/pandas-dev/pandas/jobs/455255389#L4369

```
Exception occurred:
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/git/cmd.py", line 825, in execute
    raise GitCommandError(command, status, stderr_value, stdout_value)
git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
  cmdline: git shortlog -s v0.9.0..v0.10.0
  stderr: 'fatal: ambiguous argument 'v0.9.0..v0.10.0': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]''
The full traceback has been saved in /tmp/sphinx-err-hzfqasu4.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
```

I'm not sure why that is. Perhaps something with how the repo is cloned.

Anyway, this will work around the error.

```
/Users/taugspurger/sandbox/pandas/doc/source/whatsnew/v0.10.0.rst:450: WARNING: Cannot find contributors for range 'v0.9.0..v0.1.0'
looking for now-outdated files... none found
```
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`doc/sphinxext/contributors.py`](https://github.com/TomAugspurger/pandas/blob/d09998eccf91b31534d82503bad7e489849bc257/doc/sphinxext/contributors.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23714?src=pr&el=h1) Report
> Merging [#23714](https://codecov.io/gh/pandas-dev/pandas/pull/23714?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69203636c2f661bbc167a5a4fc91ea7d30ff0b67?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23714/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23714?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23714   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51336    51336           
=======================================
  Hits        47357    47357           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23714?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23714?src=pr&el=footer). Last update [6920363...d09998e](https://codecov.io/gh/pandas-dev/pandas/pull/23714?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://travis-ci.org/pandas-dev/pandas/jobs/455318447#L2203

Ignoring the Azure failed build.

Unfortunately, not even the 0.23.4..HEAD range is being picked up, but that's OK I think.

<details>

```
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.10.0.rst:450: WARNING: Cannot find contributors for range 'v0.9.0..v0.10.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.10.1.rst:232: WARNING: Cannot find contributors for range 'v0.10.0..v0.10.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.11.0.rst:354: WARNING: Cannot find contributors for range 'v0.10.1..v0.11.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.12.0.rst:528: WARNING: Cannot find contributors for range 'v0.11.0..v0.12.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.13.0.rst:1297: WARNING: Cannot find contributors for range 'v0.12.0..v0.13.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.13.0.rst:996: WARNING: duplicate label release.bug_fixes-0.13.0, other instance in /home/travis/build/pandas-dev/pandas/doc/source/release.rst[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.13.1.rst:391: WARNING: Cannot find contributors for range 'v0.13.0..v0.13.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.13.1.rst:312: WARNING: duplicate label release.bug_fixes-0.13.1, other instance in /home/travis/build/pandas-dev/pandas/doc/source/release.rst[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.14.0.rst:1071: WARNING: Cannot find contributors for range 'v0.13.1..v0.14.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.14.1.rst:293: WARNING: Cannot find contributors for range 'v0.14.0..v0.14.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.15.0.rst:1240: WARNING: Cannot find contributors for range 'v0.14.1..v0.15.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.15.1.rst:328: WARNING: Cannot find contributors for range 'v0.15.0..v0.15.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.15.2.rst:262: WARNING: Cannot find contributors for range 'v0.15.1..v0.15.2'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.16.0.rst:699: WARNING: Cannot find contributors for range 'v0.15.2..v0.16.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.16.1.rst:489: WARNING: Cannot find contributors for range 'v0.16.0..v0.16.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.16.2.rst:189: WARNING: Cannot find contributors for range 'v0.16.1..v0.16.2'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.17.0.rst:1191: WARNING: Cannot find contributors for range 'v0.16.2..v0.17.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.17.1.rst:226: WARNING: Cannot find contributors for range 'v0.17.0..v0.17.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.18.0.rst:1314: WARNING: Cannot find contributors for range 'v0.17.1..v0.18.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.18.1.rst:716: WARNING: Cannot find contributors for range 'v0.18.0..v0.18.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.19.0.rst:1588: WARNING: Cannot find contributors for range 'v0.18.1..v0.19.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.19.1.rst:83: WARNING: Cannot find contributors for range 'v0.19.0..v0.19.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.19.2.rst:104: WARNING: Cannot find contributors for range 'v0.19.1..v0.19.2'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.20.0.rst:1755: WARNING: Cannot find contributors for range 'v0.19.2..v0.20.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.20.2.rst:149: WARNING: Cannot find contributors for range 'v0.20.0..v0.20.2'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.20.3.rst:82: WARNING: Cannot find contributors for range 'v0.20.2..v0.20.3'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.21.0.rst:1200: WARNING: Cannot find contributors for range 'v0.20.3..v0.21.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.21.1.rst:193: WARNING: Cannot find contributors for range 'v0.21.0..v0.21.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.22.0.rst:265: WARNING: Cannot find contributors for range 'v0.21.1..v0.22.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.23.0.rst:1435: WARNING: Cannot find contributors for range 'v0.22.0..v0.23.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.23.1.rst:161: WARNING: Cannot find contributors for range 'v0.23.0..v0.23.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.23.2.rst:134: WARNING: Cannot find contributors for range 'v0.23.1..v0.23.2'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.23.3.rst:28: WARNING: Cannot find contributors for range 'v0.23.2..v0.23.3'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.23.4.rst:58: WARNING: Cannot find contributors for range 'v0.23.3..v0.23.4'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.24.0.rst:1426: WARNING: Cannot find contributors for range 'v0.23.4..HEAD'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.4.x.rst:81: WARNING: Cannot find contributors for range 'v0.4.1..v0.4.3'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.5.0.rst:65: WARNING: Cannot find contributors for range 'v0.4.0..v0.5.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.6.0.rst:78: WARNING: Cannot find contributors for range 'v0.5.0..v0.6.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.6.1.rst:58: WARNING: Cannot find contributors for range 'v0.6.0..v0.6.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.0.rst:322: WARNING: Cannot find contributors for range 'v0.6.1..v0.7.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.1.rst:52: WARNING: Cannot find contributors for range 'v0.7.0..v0.7.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.2.rst:49: WARNING: Cannot find contributors for range 'v0.7.1..v0.7.2'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.3.rst:118: WARNING: Cannot find contributors for range 'v0.7.2..v0.7.3'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.8.0.rst:293: WARNING: Cannot find contributors for range 'v0.7.3..v0.8.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.8.1.rst:58: WARNING: Cannot find contributors for range 'v0.8.0..v0.8.1'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.9.0.rst:114: WARNING: Cannot find contributors for range 'v0.8.1..v0.9.0'[39;49;00m
[31;01m/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.9.1.rst:177: WARNING: Cannot find contributors for range 'v0.9.0..v0.9.1'[39;49;00m
```

</details>