If user enters buganizer bug prefixed with b/, warn user that b: is
expected.

R=apolito@google.com

Bug: 1226474
Change-Id: Ifbced843f9eecc00560cc273ae06bff9d435a815
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3016587Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>