Added exception handler for git repo init

If GitPython failed to init a git repo, currently the error message is
```
Exception No such file or directory occurred in file server update
```
from https://github.com/saltstack/salt/blob/00faa5f/salt/master.py#L232, and the message is actually coming out of `subprocess` module, as an OSError exception.

We should give user a clear hint that this error occurred precisely in GitPython, which should emit an meaningful exception here. And probably, the cause is GitPython cannot find the command line tool of git.