Headers currently have the format: `** Presubmit ${name} **`. Make these
more helpful by including the number of total errors there are.

Bug: 1341987
Change-Id: Ib41a133c31568d4264d73c14350f0bbd9590356a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3756169Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>