typing: correct the signature of error.CommandError
There's a place in `mercurial.dispatch._parse()` that passes None if a parse
error happens before the command can be parsed out, and casting the error to
bytes works fine because the command and message fields are apparently ignored.
Likewise, TortoiseHg similarly passes None for the same reason.
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).