Create method that exposes an exception.

This is the core feature of Fishbowl. Example usage:

    Throwable exception = exceptionThrownBy(
         () -> throw new NullPointerException());
Let tests fail if code thrown no exception.

Usually Fishbowl is used for testing that a piece of code throws an
exception. Hence it is a failure if the code doesn't throw an
exception. In this case Fishbowl throws a `ExceptionNotThrownFailure`
that causes the test to fail because it is a subclass of
`AssertionError`.