I'm writing a recipe that makes heavy use of gsutil's ls and cat, and
some of the calls are expected to fail. So this allows the user to make
those that fail do so as normal steps.

R=iannucci, tandrii

Bug: 866062
Change-Id: I4c9cdcb77c00afd4ab8fefd146f22fa94fcc37d7
Reviewed-on: https://chromium-review.googlesource.com/c/1325332Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>