hgweb: add a `message` attribute to `hgweb.common.ErrorResponse`

Authored by sheehan.


hgweb: add a message attribute to hgweb.common.ErrorResponse

This fixes a Python 3 bug where hgweb assumes an Exception
subclass will have a .message attribute after running
Exception.__init__.[1] The Python 3 way to get this info would
be e.args[0], but adding a new named attribute is more
ergonomic in my view.

[1] https://www.mercurial-scm.org/repo/hg-committed/file/6ccf539aec71/mercurial/hgweb/hgwebdir_mod.py#l459

Differential Revision: https://phab.mercurial-scm.org/D6840