code: function requireAttribute(filter, keyword, attribute) {
    if (!filter[attribute]) {
      return Bluebird.reject(new BadRequestError(`"${keyword}" requires the following attribute: ${attribute}`));
    }
  
    return Bluebird.resolve();
  }
docs: Verifies that "filter.attribute' exists
  @param filter
  @param keyword
  @param attribute
  @returns {Promise}