If AIPerService::add_queued_to fails because a new request is throttled,
then do not add the request to the end of the queue, nor remove it from
the queue: do nothing: it makes no sense to move the request to the back
because they all belong to the same service and all of them will be
either throttled or not.
Note: Still need to fix that in this case we should look in queues of
other services.
67e88561dc