cras_dbus_control: ensure send_*_reply errors are handled

By using __attribute__((warn_unused_result)).

BUG=None
TEST=CQ

Change-Id: I851ac9ed8449791ed5b32541c9e0c4dd34939d7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/5198328
Tested-by: Li-Yu Yu <aaronyu@google.com>
Commit-Queue: Curtis Malainey <cujomalainey@chromium.org>
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
Auto-Submit: Li-Yu Yu <aaronyu@google.com>
Tested-by: chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com <chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com>