Also, add a couple of macros to handle error cases.

R=adamk@chromium.org

Bug: chromium:744292
Change-Id: I5dcb19ce67ec1aa4318d68d973d304cb07a65b80
Reviewed-on: https://chromium-review.googlesource.com/575394
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46722}