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 
{
        $this->expectCallable(
                () ==> {  throw new \HackPack\HackUnit\Core\ExpectationException("Message");
        })->toThrow('\HackPack\HackUnit\Core\ExpectationException', 'Message');
}
```
I still think this should be if($expectedMessage !== null && ...)

The type checker complains that "isset cannot be used in a completely type safe way and so is banned in strict mode (Typing[4016])"