When there are missing input files, upload_to_google_storage.py
should not display 'Success'.

Bug: 920654
Change-Id: I1bc7ba6e8eb48ea28b634da01e1be9a80af7b719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2647870Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: John Chen <johnchen@chromium.org>