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Receiving the unmount error code will let Chrome properly handle various
types of errors, and produce better logging.

BUG=873903

Change-Id: I792a806440be6725ceba0663afc4b4ee3069d9bd
Reviewed-on: https://chromium-review.googlesource.com/1179109Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Anand Mistry <amistry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584348}