Document exceptions in constants module

[willkg]

This adds docstrings to the exceptions in the constants module for #307.

[willkg]

I'm not as well-versed in the html5lib module as others, so please feel free to comment on improvements that should be made to the suggested text.

[willkg]

@gsnedders @jgraham Can one of you review this please?

[gsnedders]

Let's be more specific:

Raised when the current tree is unable to represent the input data

[willkg]

I'll change this. Thanks!

[gsnedders]

This should really be internal; it should never be raised through any public interface.

[willkg]

The way the docs are structured, it's difficult to make this not show up in the docs. I'll take away the docstring, but that's how it's going to have to live for 1.0.

[gsnedders]

We should probably just _ prefix it, I guess?

[willkg]

Sure! If that's important, someone who is not me can write up an issue for making those changes and make them. If that makes it in time for 1.0, that's cool. Otherwise I'm not worrying about this.

[willkg]

^^^ That adjusts the docstrings for those two classes per @gsnedders' comments.

[willkg]

AppVeyor is failing with this:

Collecting py>=1.5.0 (from pytest)
  Could not find a version that satisfies the requirement py>=1.5.0 (from pytest) (from versions: 0.9.1, 0.9.2, 1.0.0, 1.0.1, 1.0.2, 1.1.0, 1.1.1, 1.2.0, 1.2.1, 1.3.0, 1.3.1, 1.3.2, 1.3.3, 1.3.4, 1.4.0, 1.4.1, 1.4.2, 1.4.3, 1.4.4, 1.4.5, 1.4.6, 1.4.7.dev3, 1.4.7, 1.4.8, 1.4.9, 1.4.10, 1.4.11, 1.4.12, 1.4.13, 1.4.14, 1.4.15, 1.4.16, 1.4.17, 1.4.18, 1.4.19, 1.4.20, 1.4.21, 1.4.22, 1.4.23, 1.4.24, 1.4.25, 1.4.26, 1.4.27, 1.4.28, 1.4.29, 1.4.30, 1.4.31, 1.4.32, 1.4.33, 1.4.34)
No matching distribution found for py>=1.5.0 (from pytest)

That has nothing to do with this PR, so I'm going to ignore it.

[gsnedders]

We also have a Travis failure, seemingly because us testing against six==1.9 and pytest now requiring six>=1.10. But yeah, unrelated.

[willkg]

Thank you!