message: check that promise actually throws
diff: @@ -108,11 +108,12 @@ and enable the feature in your `.babelrc` file.
  Errors can be handled in the standard JavaScript way: Either using `.catch()`
  directly on a Promise or through `try-catch` when using async/await. Note that
  -if a Promise throws and the error is not handled, the test will fail.
  +if a Promise throws and the error is not handled, the test will fail. `expect.assertion(1)` makes sure that expectation was checked once. In this example it will fail if promise was resolved without throwing.
  ```js
  // Testing for async errors can be done using `catch`.
  it('tests error with promises', () => {
  + expect.assertions(1)
  return user.getUserName(3)
  .catch(e => expect(e).toEqual({
  error: 'User with 3 not found.',
  @@ -121,6 +122,7 @@ it('tests error with promises', () => {
  // Or try-catch.
  it('tests error with async/await', async () => {
  + expect.assertions(1)
  try {
  await user.getUserName(2);
  } catch (object) {
  