fix(repo): prevent error messages from being swallowed
fix(repo): prevent error messages from being swallowed

This issue must have rosen from `to_progress_instance()`
being inserted in a spot where `None` was a legit value.

Fixes #462

gitpython-developers/gitpython,jeblair/GitPython,gitpython-developers/GitPython,gitpython-developers/gitpython,breavo/GitPython,gitpython-developers/GitPython,expobrain/GitPython,nvie/GitPython,nvie/GitPython,jeblair/GitPython,breavo/GitPython,expobrain/GitPython