Added a dummy class for HTMLParserError.

Just looking for someone to +1 this approach.
> Does Django call that method?

Looks like it doesn't on 3.5:

* https://github.com/timgraham/django/blob/htmlparser/django/utils/html_parser.py#L7-10
* https://github.com/timgraham/django/blob/htmlparser/django/utils/html_parser.py#L95

> All the tests pass after this change except for some ImportError exceptions being reported like in tests/migrations/faulty_migrations/import_error

It could be related to http://bugs.python.org/issue7559