Before, presubmit_support would fail with not very useful stacktrace if
Gerrit returns 404, which is usually due to missing/invalid credentials.

This CL fixes that and improves the exception message, and also improves
logic in git_cl.

R=agable@chromium.org
BUG=

Change-Id: Iae8f0c24422c46af70929c7d5d71993164887511
Reviewed-on: https://chromium-review.googlesource.com/409650
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>