(PECL gearman >= 0.5.0)
GearmanClient::setFailCallback — Set callback for job failure
Sets the callback function to be used when a task does not complete successfully.
Note:
The callback will only be triggered for tasks that are added (e.g. by calling GearmanClient::addTask()) after calling this method.
callbackA function or method to call. It should return a valid Gearman return value.
   If no return statement is present, it defaults to GEARMAN_SUCCESS.
  
taskThe task this callback is called for.
context
      Whatever has been passed to GearmanClient::addTask() (or equivalent method) as context.
     
