Acknowledgement Message Format

Dear Experts,

I have a BPM in my scenario. The end step of this BPM is Synchronous Send.

In the response message i want to configure a structure which will tell me if the message processing is successsful or not.

Suppose if the AE is not able to send the message to the end system, then it will throw an error back to the BPM. Now my requirement is to catch this error message, so how to proceed on this..please help

Thanks