Remove cancel message in favor of CANCELED error type.

Most code doesn't care to distinguish between an error and cancelation,
as both abort the transfer. The few places that do care (e.g., the code
that decides whether to show an error to the user) can explicitly check
the type. This also allows cancellation to be easily propagated through
the error path from the source, instead of requiring a secondary
cancellation path.

Bug: 679313
Change-Id: I3c90ff55e7a58043535244a9d92cfc9e6814a4e6
Reviewed-on: https://chromium-review.googlesource.com/c/1437657
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626294}
2 files changed