Looking at this. Off hand, not sure how to force the failure ... maybe creating a conflict in what is in the response message body and where it gets mapped on the clipboard ... missing field, wrong data type, etc.
However, I think this would result in a failure further downstream in the communication, once it begins to process Pega Rule content.
However, I am trying to break something in the initial connection. I want to see the default Error Property content on failure, so I can search on the string, and take necessary action when the communication failure happens.
Not sure what your level of "control" is for the Connector and Service itself, but I would imagine a failed authentication should cause the "Error handler flow" to execute as well as set the message and status properties.
Or maybe configuring the "Security setting" of the Connector may cause the failure you are trying to invoke. I have never used the "Security settings" myself, but I'd imagine it's the minimum security required between the Connector and Service.