stop using bare except:

[Jack O'Connor]

A more recent version of flake8 has started failing on bare except, which is very reasonable. https://github.com/buildinspace/peru/tree/bareexcept is a first attempt to get rid of them. That fails a git plugin test, because previously we were using bare except to suppress a sys.exit call. (Horrifying!) We need to clean up the shell-out helper a little bit, so that callers that want to handle errors have the option of doing that.