[swarming] do not try to trap CommitError in cancel_task
[swarming] do not try to trap CommitError in cancel_task

It wasn't returning a correct value, a string instead of a tuple. So it caused a
ValueError instead (too many values to unpack) which isn't informative.

CommitError are expected to happen at scale so it's easier to filter these.

Change-Id: I00db2f9d14c525cf14eda1c3364793d688f4130c
Reviewed-on: https://chromium-review.googlesource.com/1104388
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py