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).