openWebHook() {
    if (this.isPolling()) {
      return Promise.reject(new errors.FatalError('WebHook and Polling are mutually exclusive'));
    }
    if (!this._webHook) {
      this._webHook = new TelegramBotWebHook(this);
    }
    return this._webHook.open();
  }
Open webhook.
Multiple invocations do nothing if webhook is already open.
Rejects returned promise if Polling is being used by this instance.

@return {Promise}