Log the 'code' attribute on validation failure

With a CAS server that requires registering service URLs, configuring the
client with an invalid service URL causes a failure with the code
'INVALID_SERVICE' and contents "ticket ##### not recognized", e.g.:
<cas:authenticationFailure code="INVALID_SERVICE">
        ticket 'ST-7ED125C[...]' not recognized
    </cas:authenticationFailure>

Logging the code is extremely helpful in tracking down the problem, much more
so than just "ticket not recognized".