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).
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable