typing: add more type hints to the errors module

These were found by looking for Any types in the generated *.pyi file after
running the tests. There are some more complicated types that I'm not sure of,
and am leaving untyped. I also can't figure out how to get self.hint to be
anything other than Any in most classes.

Differential Revision: https://phab.mercurial-scm.org/D11719