forall: handle missing project refs better

If the project exists, but the ref the manifest wants doesn't exist,
don't throw an error (and abort the process in general).  This can
come up with a partially synced tree: the manifest is up-to-date,
but not all the projects have yet been synced.

Bug: https://crbug.com/gerrit/14289
Change-Id: Iba97413c476544223ffe518198c900c2193a00ed
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/301262


Reviewed-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>