Redirect if no WLS-Response is present

If you go to `/raven/auth` manually you currently get three notices:

* Warning: Notice: Undefined index: WLS-Response in raven_auth()
* Warning: Suspicious login attempt denied and logged.
* Warning: Notice: Undefined index: WLS-Response in raven_auth()

This checks to see the the `WLS-Response` query string parameter is set, and, if not, immediately redirects to the failure page.