How Do You Handle a POST Rest Web Service Operations When the Response Message can be in 2 Different

We created a POST Rest Web Service Operation, which is not a Provider Service, that is published from our PeopleSoft HCM system to Service Now.

When Service Now sends back a Successful Response Message, it sends back the Response Message like the following message.  This is working.

If the Request Message sent to Service Now contains a value that they do not have in their system (such as a department ID), then they send the following Error Response Message.  

When the Error Response Message is returned, the Service Operation will fail with the error message, ‘400 Bad Request’.

How keep the Service Operation from failing when we get the Error Response Message and be able to parse out the message?