presubmit --all should succeed regardless of the current state of the
tree. Otherwise a presubmit --all bot will be flaky due to circumstances
beyond its control.

This is a logical follow-on to crrev.com/c/3628368 which turned some
other errors into warnings for presubmit --all.

Bug: 1309977
Change-Id: I6d26708f8c47916cf015dec75e27833db7ca465b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3696070
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>