DOC: Making the build fail if the doc build fails
The doc build is failing silently if sphinx fails. This PR makes the CI red when something goes wrong building the docs.

CC: @jreback 

It has been tested here: https://travis-ci.org/pandas-dev/pandas/jobs/468243202
Hello @datapythonista! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`doc/make.py`](https://github.com/datapythonista/pandas/blob/cb8f940eca24fa32cd8934bf5f9ae0b87c4ae98a/doc/make.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=h1) Report
> Merging [#24292](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ca85a41242af245b32f6c62580b072d1685342d4?src=pr&el=desc) will **decrease** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24292/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24292      +/-   ##
==========================================
- Coverage   92.28%   92.22%   -0.06%     
==========================================
  Files         162      162              
  Lines       51827    51824       -3     
==========================================
- Hits        47827    47795      -32     
- Misses       4000     4029      +29
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (-0.07%)` | :arrow_down: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/24292/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `0% <0%> (-86.37%)` | :arrow_down: |
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/24292/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `76.92% <0%> (-7.7%)` | :arrow_down: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/24292/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `72.09% <0%> (-0.78%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24292/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.41% <0%> (-0.07%)` | :arrow_down: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/24292/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.56% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24292?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/24292?src=pr&el=footer). Last update [ca85a41...8ad2ff1](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=h1) Report
> Merging [#24292](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ca85a41242af245b32f6c62580b072d1685342d4?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/24292/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24292   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files         162      162           
  Lines       51827    51827           
=======================================
  Hits        47827    47827           
  Misses       4000     4000
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24292?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/24292?src=pr&el=footer). Last update [ca85a41...8ad2ff1](https://codecov.io/gh/pandas-dev/pandas/pull/24292?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @datapythonista 