If git cl upload fails due to a private key being detected,
print more useful message for git-cl users.

R=aravindvasudev@google.com

Fixed: 1201742
Change-Id: Ida5c9c9788058dc17c1c50034c9b0b8831cfbd20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3453203
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>