Add stderr logging to 'git fetch' operation

This part of the code sees frequent failures on our internal RTD for random stuff like "user put in a public Github https URL for a private repo" (causes password prompt on stdin), "user checked in a pyc file for a sphinx extension" (so builds mutate the git repo in a non-pullable/fetchable fashion), etc.

Git returns code 128 for almost any error condition so having just the code is not useful :)