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');
}
```
Sorry... Because of the compression of unchanged lines it was hard to follow the braces. It now looks to me like it is in the catch statement.