py3: fix curses chunkselector fallback (when diffs are too large) on py3 Previously we showed the message using Exception.message, which is removed in py3. Since crecordmod.fallbackerror inherits from error.Abort, we can just use `b'%s' % exception` to print the message. This does not print the hint, but that's fine - we don't set one. We inherit from error.Abort so that if a codepath doesn't handle fallback specially, it exits to the terminal with a sane message instead of an unknown exception error. Differential Revision: https://phab.mercurial-scm.org/D7912