demonstrate that `.catch(err => Promise.reject(err))` is redundant

es6-promise-catch-behavior.js

'use strict';

(new Promise((resolve, reject) => {
    resolve('success');
})).then(x => {throw 'thrown error in then, no catch'});

(new Promise((resolve, reject) => {
    resolve('success');
})).then(x => {throw 'thrown error in then, with catch'})
    .catch(err => Promise.reject(err));

// both result in UnhandledPromiseRejectionWarning
// (node 6.12.3)