stdout print statement may interfere with automated tooling. Instead,
this prints the warning to stderr. See: https://crrev.com/c/3556588

R=gavinmak@google.com
Change-Id: I3fb72b7ab4afe42cb83ea5e506292f3e6e5ea8d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3561445
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>