Nagios Website check is successful even if the website has problems

i tried a "leap deploy local", after which the webapp didn't start properly (see #1628 (closed)). It doesn't give an html error, but displays "We're sorry, but something went wrong".

So nagios should check for a string to show up.

(from redmine: created on 2013-02-01, closed on 2013-11-19, relates #2718)