SAML20/AuthnResponse: Send correct response code for errors.

A small bug with the previously committed code, which could result in a
response being sent with a status code of success when it was actually
an error. The error was in the code which was included for backwards-
compatibility.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1583 44740490-163a-0410-bde0-09ae8108e29a