code: def callback(self, task, cb, error_cb, *args, **kw):
          
          threaded(self._do_cb, (task, cb, error_cb) + args, kw)
docs: Threads the task, then runs a callback on success or an error callback
          on fail.
          
          cb       -- The function to be called with a successful result.
          error_cb -- The function to be called when an error occurs.
          
          For information on `task`, *args, and **kw, see work().