rejectAdd(storeName) {
    return Promise.reject(new shaka.util.Error(
        shaka.util.Error.Severity.CRITICAL,
        shaka.util.Error.Category.STORAGE,
        shaka.util.Error.Code.NEW_KEY_OPERATION_NOT_SUPPORTED,
        'Cannot add new value to ' + storeName));
  }
@param {string} storeName
@return {!Promise}
@protected