claw_git_status.py shouldn't raise an exception if not a git repository

Currently `src/python/clawutil/claw_git_status.py` raises an exception of run from a directory that is not a git clone of clawpack (e.g. a version installed from a tar file).  Instead it should just print out that fact.  

This was throwing an error in a regression test that had `CLAW_GIT=True` in the `Makefile` and it would be nice to encourage that in general.