Presubmit message types were printed in an inconsistent order, generally
determined by which type was encountered first. This unpredictability
can be confusing when comparing runs. This change enforces a consistent
order, in increasing order of severity. ERRORS always go last so that
they will be the most visible when running presubmits locally.

This also adds a "There were Python %d presubmit errors." message to
the end to make it easier to tell when there were presubmit errors.
This is particularly useful when manually running presubmits.

Bug: 1309977
Change-Id: Ib2b73c7625789bad5b21ae12abf238b746cd11e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3668724Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>