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