Given the current code, I don’t believe that there is a way to change the message. Parts of that message are hardcoded (https://github.com/istio/istio/blob/55a1771204c5d996c58db69434f188019a6aa01b/mixer/adapter/list/list.go#L118, https://github.com/istio/istio/blob/55a1771204c5d996c58db69434f188019a6aa01b/mixer/pkg/runtime/dispatcher/session.go#L386).
You could perhaps change the listchecker to take a configurable message. But, then you would also have to change the client code to pull out the message that you want to return (iirc, https://github.com/istio/proxy/blob/8455b9eb514c1a527fc4447951af00acd83104f5/src/istio/mixerclient/check_cache.cc#L193).