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

[rjleveque]

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.