grep: handle errors gracefully

If `git grep` fails in any project checkout (e.g. an incomplete
sync), make sure we print that error clearly rather than blowing
up, and exit non-zero in the process.

Bug: https://crbug.com/gerrit/11613
Change-Id: I31de1134fdcc7aaa9814cf2eb6a67d398eebf9cf
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239237
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>