protected function getErrorMessage($response, $alternativeError = 'Unknown error.')
    {
        $xml = $this->parseResponse($response);
        if ($xml && $xml->Message) {
            return "[" . $response->getStatusCode() . "] " . (string)$xml->Message ."\n" . (string)$xml->AuthenticationErrorDetail;
        } else {
            return $alternativeError;
        }
    }