worker: fixed infinite loop on send failure

The problem here is when qr_task_send() returns an error, the following error handler will attempt to cancel all tasks that were started on the same connection, but that will only work for the first task (which is finished), the qr_task_on_send() will have no effect on tasks in progress as the passed handle is NULL, and the task->finished is false, thus looping infinitely.

The solution here is to let the rest of the tasks complete, even though sending answer back will fail (which is fine).