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 de
  checking for parse errors
  Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
  warning: please have a space before :hg: