Look for reponse_code in its correct placement, add message if found

response_code was being looked for in
<code class='php'>$response['body']['response_code']</code>
instead of
<code class='php'>$response['body']['data']['response_code']</code>

Also, added the error explanation as consts in ```Services_Paymill_Apiclient_Interface``` so the error messages are more friendly for logging.

```CurlTest.php``` updated accordingly.