How to use the knex.Promise.reject function in knex

// If the function is a generator, wrap it using Promise.coroutine.
    if (isGenerator(args[0])) {
      args[0] = Promise.coroutine(args[0]);
    }

    return knex.transaction.apply(knex, args);
  } else {
    // The last argument should be the callback and all other Model subclasses.
    let callback = _.last(arguments);
    let modelClasses = _.take(arguments, arguments.length - 1);
    let i;

    for (i = 0; i < modelClasses.length; ++i) {
      if (!isSubclassOf(modelClasses[i], Model)) {
        return Promise.reject(new Error('objection.transaction: all but the last argument should be Model subclasses'));
      }
    }

    let knex = _.first(modelClasses).knex();
    for (i = 0; i < modelClasses.length; ++i) {
      if (modelClasses[i].knex() !== knex) {
        return Promise.reject(new Error('objection.transaction: all Model subclasses must be bound to the same database'));
      }
    }

    // If the function is a generator, wrap it using Promise.coroutine.
    if (isGenerator(callback)) {
      callback = Promise.coroutine(callback);
    }

    return knex.transaction(trx => {

transaction.start = function (modelClassOrKnex) {
  let knex = modelClassOrKnex;

  if (isSubclassOf(modelClassOrKnex, Model)) {
    knex = modelClassOrKnex.knex();
  }

  if (!_.isFunction(knex.transaction)) {
    return Promise.reject(new Error('objection.transaction.start: first argument must be a model class or a knex instance'));
  }

  return new Promise((resolve, reject) => {
    knex.transaction(trx => {
      resolve(trx);
    }).catch(err => {
      reject(err);
    });
  });
};