py3: use stringutil.forcebytestr() instead of str()
We need to convert errors to bytes using stringutil.forcebytestr()
Differential Revision: https://phab.mercurial-scm.org/D3324
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.