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This is a continuation from crosreview.com/737056. When Stop() discovery
fails because Discovering property changes to false, that means all the
discovery sessions have been stopped bluez, so Stop() is expected to
fail and there is no need to log this as error.

BUG=777953
TEST=Reproduce the case and check that device event log is categorized
as DEBUG instead of ERROR.

Change-Id: I41f6e3e83e9f6377bf28f4bb2210be34bdf1c732
Reviewed-on: https://chromium-review.googlesource.com/747603Reviewed-by: Miao-chen Chou <mcchou@chromium.org>
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513022}