[Background Fetch] Report DatabaseTask errors to scheduler.

The scheduler assumes that all database tasks succeeded and continues
accordingly. This change propagates the errors, and if there are any,
the scheduler will abandon the fetch.

Change-Id: I813ab23107369799ada4cc39a5eee9944433ed44
Reviewed-on: https://chromium-review.googlesource.com/1228015
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592387}
11 files changed