CAS-1396: Fixed incorrect message in the proxy failure response

to correctly indicate the reason for failure in case of unauthorized proxy access.

https://issues.jasig.org/browse/CAS-1396

Expected result now is:

```xml
<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
	<cas:authenticationFailure code='UNAUTHORIZED_SERVICE_PROXY'>
		The supplied service &#039;https://mmoayyed/exampleapp/Restricted/AuthenticatedUsersOnly/default.aspx&#039; is not authorized to use CAS proxy authentication.
	</cas:authenticationFailure>
</cas:serviceResponse>
```