somePromise().then(
  function onSuccess (res) {
    // stuff happens, but oh no!
    // an error is thrown in here!
  },
  function onError (err) {
    // request-only error handler
  }
);

somePromise()
  .then(function onSuccess (res) {
    // stuff happens, but oh no!
    // an error is thrown in here!
  })
  .catch(function onError (err) {
    // yay! The error thrown in the function above
    // can be handled here or rethrown to be handled elsewhere.
  });