forall: exit 1 if we skip any repos

If a repo doesn't exist (e.g. an incomplete sync), make sure we exit
non-zero when they get skipped.

Change-Id: Ifff711e374416b1e6b9b8da4fdc6f14b27ced450
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239235
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>