Test for failure, but don't test the error message

The push tests currently look at the result from core git.  Other git servers may not necessarily report the same message, so let's not fail a test just because we're talking to something odd and hyper-verbose.

For example, one server's `funny refname` is another server's `TF401021: 'refs/heads/b2~1' is not a valid name for a Git ref in this context.  Abort, Retry, Fail, Ignore?`