Git related patches

Following patches fix Git support in Frescobaldi, in particular error handling. When I ran Frescobaldi in my repository, it crashed as soon as I selected Git in the menu. It turns out Frescobaldi cannot deal with STGit branches. They are mistaken for remote branches. The error handling in the Git related code was quite bad. One exception was simply undefined. Syntax errors I made while fixing the code were interpreted as if Git wasn't installed. The message about missing Git was quite unfriendly to the users.