handleError_(e) {
    // If the maximum number of retries has been reached.
    if (!this.retry()) {
      this.event_ = e;
      this.markCompleted_();
    }
  }
Handles the error of a request. If the request has not reach its maximum
number of retries, then it lets the request retry naturally (will let the
request hit the READY state). Else, it dispatches the ERROR event.
@param {goog.events.Event} e An event to handle.
@private