code: function(worker) {
        var data = getWorkerNamespacedData(worker);
  
        Object.keys(data.pending).forEach(function(seqNum) {
          var item = data.pending[seqNum];
          delete data.pending[seqNum];
          if (item[0].monitored) {
            item[1](new Error("Undeliverable message: worker died before we could get acknowledgement"));
          }
        });
      }
docs: If a worker dies, fail any monitored ack deferreds.