git_summary.py gives ugly error if someone uses zip from github

[reticulatedpines]

git_summary.py errors like this if someone downloads zip of the repo and tries to build it:
fatal: not a git repository (or any parent up to mount point /mnt)

We should either not fail, and make a build with a "not git managed" style message for the user, or, we should print a nicer message explaining why it failed and possible fixes.

See the related https://github.com/reticulatedpines/magiclantern_simplified/issues/17