[ticket/12936] Assert that status code is 200

https://tracker.phpbb.com/browse/PHPBB3-12936
PHPBB3-12936
I think I said this before but it would be nice to know more than just that the status code is unexpected. Maybe check if there was a message output in addition to the status code and output to the logs? Right now it's just telling you that you have a 503 but not actually what the page says during that 503.