This updates djblets.webapi.errors to include type hints and modern

documentation. This class is ancient, and was not up to our current

standards.
This is in preparation for upcoming work on error support.
There is one small feature addition in that with_override() and

with_message() will now do the right thing if called on a WebAPIError

subclass. While not a pattern we currently explicitly support, this is

more correct and helps future-proof the class.

Unit tests pass.