Add option for acknowledging failed tasks (globally and per-task)

If a task fails with an exception, it is retried until it
succeeds. This is contrary to what is said in the documentation: under
the "Architecture" section, heading "Broker" it says that even when a
task errors, it's still considered a successful delivery. Failed tasks
never get acknowledged however, thereby being retried after the
timeout period. See also issues #238 and #194.

This patch adds an option to acknowledge failures, thereby closing
issue #238. Issue #194 would require some more work. The default of
this option is set to `False`, thereby maintaining backwards
compatibility.

django_q/cluster.py

@@ -325,12 +325,12 @@ def monitor(result_queue, broker=None):
             save_cached(task, broker)
         else:
             save_task(task, broker)
-        # acknowledge and log the result
+        # acknowledge result
+        ack_id = task.pop('ack_id', False)
+        if ack_id and (task['success'] or task.get('ack_failure', False)):
+            broker.acknowledge(ack_id)
+        # log the result
         if task['success']:
-            # acknowledge
-            ack_id = task.pop('ack_id', False)
-            if ack_id:
-                broker.acknowledge(ack_id)
             # log success
             logger.info(_("Processed [{}]").format(task['name']))
         else: