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 was thinking that, but that would allow null to be passed in?

An easier thing which would solve the issue you've got elsewhere is to default this to an empty string and then later on do a check to see if it's an empty string.