Catch the retcode for all commands.

* bzrlib/tests/script.py:
(ScriptRunner._check_output): Fix typos in comment.
(ScriptRunner.do_bzr, ScriptRunner.run_command): Get and returns
the retcode.
(ScriptRunner.do_cat, ScriptRunner.do_cd, ScriptRunner.do_mkdir):
Returns a retcode.

* bzrlib/tests/__init__.py:
(TestCase._run_bzr_core): Also returns the retcode for inspection
by callers.