Some DRY cleanup/error reporting improvement for gitfs

This commit does some DRY cleanup in the __virtual__ function, as well
as some of the provider verification code. In addition, it fixes a
couple minor aesthetic error reporting issues.
Merge pull request #10284 from terminalmage/gitfs-dry-cleanup-2014.1

Some DRY cleanup/error reporting improvement for gitfs (2014.1 branch)