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When cancel scan request fails, a toast with error message will pop up,
the scan will continue, and the scan progress page will keep showing.

Bug: 1059779
Change-Id: I9ba3a7a29837f55e8b166224949be1d403e3661c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559496
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832156}