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
copyright (c) 2006-2010 David JEAN LOUIS
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