How to use the http-status.TOO_MANY_REQUESTS function in http-status

// Message to replace removed headers with in the request
var REMOVED_HEADER_MESSAGE = '[REMOVED BY SDK]';

// Range of SERVER ERROR http status codes
var HTTP_STATUS_CODE_SERVER_ERROR_BLOCK_RANGE = [
	500,
	599
];

// Timer used to track elapsed time beginning from executing an async request to emitting the response.
var asyncRequestTimer;

// A map of HTTP status codes and whether or not they can be retried
var retryableStatusCodes = {};
retryableStatusCodes[httpStatusCodes.REQUEST_TIMEOUT] = true;
retryableStatusCodes[httpStatusCodes.TOO_MANY_REQUESTS] = true;

// Retry intervals are between 50% and 150% of the exponentially increasing base amount
const RETRY_RANDOMIZATION_FACTOR = 0.5;

/**
 * Returns true if the response info indicates a temporary/transient error.
 *
 * @param {?APIRequest~ResponseObject} response The response info from an API request,
 * or undefined if the API request did not return any response info.
 * @returns {boolean} True if the API call error is temporary (and hence can
 * be retried). False otherwise.
 * @private
 */
function isTemporaryError(response) {
	var statusCode = response.statusCode;

entitiesToIndex = response.items.reduce((accumulator, item) => {
				// We currently only handle queue overrun
				if (item.index.status === httpStatus.TOO_MANY_REQUESTS) {
					const failedEntity = entities.find(
						(element) => element.bbid === item.index._id
					);

					accumulator.push(failedEntity);
				}

				return accumulator;
			}, []);

exports.rateLimitHandler = (req, res, next) => {
  const err = new APIError({
    message: 'Rate limt exceeded, please try again later some time.',
    status: httpStatus.TOO_MANY_REQUESTS,
  });
  return handler(err, req, res);
};

async function newModel(req: auth.RequestWithProject, res: Express.Response) {
    switch (req.project.type) {
    case 'text': {
        try {
            const model = await conversation.trainClassifier(req.project);
            return res.status(httpstatus.CREATED).json(returnConversationWorkspace(model));
        }
        catch (err) {
            if (err.message === conversation.ERROR_MESSAGES.INSUFFICIENT_API_KEYS) {
                return res.status(httpstatus.CONFLICT).send({ code : 'MLMOD01', error : err.message });
            }
            else if (err.message === conversation.ERROR_MESSAGES.API_KEY_RATE_LIMIT) {
                return res.status(httpstatus.TOO_MANY_REQUESTS).send({ code : 'MLMOD02', error : err.message });
            }
            else if (err.message === conversation.ERROR_MESSAGES.MODEL_NOT_FOUND) {
                return res.status(httpstatus.NOT_FOUND)
                          .send({ code : 'MLMOD03', error : err.message + ' Please try again' });
            }
            else if (err.statusCode === httpstatus.UNAUTHORIZED) {
                return res.status(httpstatus.CONFLICT)
                        .send({
                            code : 'MLMOD04',
                            error : 'The Watson credentials being used by your class were rejected. ' +
                                    'Please let your teacher or group leader know.',
                        });
            }
            else if (err.message === 'Unexpected response when retrieving service credentials') {
                return res.status(httpstatus.CONFLICT)
                    .send({