worker: fixed infinite loop on send failure

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).