I was not able to configure DataPower correctly to process HTTP backend error in trying to answer developerWorks forum posting question.
I asked for a sample config, which I got, then minimized and enhanced to do the processing required (process the HTML response body).
Find the details in the forum posting, nice minimal service is attached there:
A better service with HTTP response code dispatcher (if "200 OK" --> XML processing, else Non-XML processing) is described and attached here:
This is the stylesheet used to extract the backend response code as well as the response body, with UTF-8 validation: