fail when AssertionError expected but not thrown

When a user expected the production code to throw an AssertionError by
using ExpectedException (together with the #handleAssertionErrors
option) but the code under test didn't do so the test was not failing.
This was because the ExpectedException rule caught its own
AssertionError issued to signal that the expected exception was not
thrown.
assertion on complete error message
correct formatting
Merge pull request #583 from UrsMetz/expected-exception-fail-when-assertion-error-expected-but-not-thown

ExpectedException doesn't fail when an AssertionError is expected but not thrown