Do not push a new task on database operational error without channel

If a task is pushed again, this creates an infinite loop because the task is
appended to the transaction tasks list and so it is run again in the same
request.

issue10421
review346251002

trytond/worker.py

@@ -128,6 +128,9 @@ def run_task(pool, task_id):
             time.sleep(0.02 * retry)
         try:
             with Transaction().start(pool.database_name, 0) as transaction:
+                if not transaction.database.has_channel():
+                    logger.critical('%s failed', name, exc_info=True)
+                    return
                 task = Queue(task_id)
                 scheduled_at = dt.datetime.now()
                 scheduled_at += dt.timedelta(