BUG: Validate the justify parameter in to_html
Closes #17527

cc @TomAugspurger 
Hello @gfyoung! Thanks for updating the PR.







Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on October 03, 2017 at 18:20 Hours UTC lgtm. merge on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=h1) Report
> Merging [#17766](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2ff1241fa794231b8317ebe96b66f71dce99e0c2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17766/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17766      +/-   ##
==========================================
+ Coverage   91.23%   91.24%   +<.01%     
==========================================
  Files         163      163              
  Lines       49869    49872       +3     
==========================================
+ Hits        45499    45504       +5     
+ Misses       4370     4368       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.24% <33.33%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `96.07% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17766?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/17766?src=pr&el=footer). Last update [2ff1241...6e43b32](https://codecov.io/gh/pandas-dev/pandas/pull/17766?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback , @TomAugspurger : All green, so merging.