localrepo: fix conversion of exceptions to strings flagged by pytype
This is the same as e571fec5b606. I assume the 3 similar uses of `bytestr` in
`__getitem__()` are OK (even though pytype also flags them), because there is
apparently test coverage for 2 of the 3 cases.
Differential Revision: https://phab.mercurial-scm.org/D10131
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors