Always capture copybot warnings, even on error

BUG=None
TEST=None

Change-Id: Ide13d2bce09d2dd54be853f0b44d624388dbc363
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/recipes/+/7841309
Commit-Queue: Jeremy Bettis <jbettis@chromium.org>
Reviewed-by: Navil Perez <navil@google.com>