If git push refs/for/ errors out, inspect stdout message to detect
uploadvalidator rejection. If present, present user a solution.

R=ajp@google.com

Change-Id: I70aee7615f3d905127a31bd5679968ade75c234a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2888475Reviewed-by: Andy Perelson <ajp@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>