upload: improve no pending CL error handling

Show clearer messages and exit non-zero if there's nothing to upload.

Change-Id: Icd9c13b9b1126610a409fc13d1d11bfc66f5e802
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255834
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>