Add check for exception message

Allows for you to test your exception messages to ensure the exception has been flung at the right time.

Example

```hack
public function testThrowsException():void 
{
        $this->expectCallable(
                () ==> {  throw new \HackPack\HackUnit\Core\ExpectationException("Message");
        })->toThrow('\HackPack\HackUnit\Core\ExpectationException', 'Message');
}
```
The type checker always complains at me when I try to use isset(). I think the test you want here is !== null. I also this this entire if block should be inside the catch statement. That way it is only run when an exception is thrown. No exception, no message to test...