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

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