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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).