code: func (t *Throttler) Done(err error) {
  	if err != nil {
  		t.errsMutex.Lock()
  		t.errs = append(t.errs, err)
  		atomic.AddInt32(&t.errorCount, 1)
  		t.errsMutex.Unlock()
  	}
  	t.doneChan <- struct{}{}
  }
docs: // Done lets Throttler know that a job has been completed so that another worker
  // can be activated. If Done is called less times than totalJobs,
  // Throttle will block forever