shelve: consistently convert exception to bytes via `stringutil.forcebytestr`
The other two places in this module use this, and past experience shows that
this method does a nicer job. I'm not sure why we're converting to bytes here-
`KeyError` is built-in and will have str attrs, and `RepoLookupError` is a
subclass of the built-in `Exception` class (not `errors.Error`, which is
allegedly the baseclass for all Mercurial exceptions).
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).