tests/test-mq-qsave.t

tests: make test-worker.t pass on py2

I broke the py2 version in https://phab.mercurial-scm.org/D9287
because the `WorkerError.__bytes__()` (or `.__str__()`?) output was
different in py2 compared to py3. Part of the problem was that I
didn't propagate the status code that was passed in to the superclass
so it could get printed. This patch fixes that. I don't know how it
worked on py3 before this patch...

I also added the usual `__bytes__ = _tobytes` override for good
measure. It doesn't seem to be needed for tests to pass, though.

Differential Revision: https://phab.mercurial-scm.org/D9377

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state