Evidently the activity status of the Service Activity can't be interrogated from the When rules you use in the Response Mapping configuration.
Note that the Tracer shows the Step Page of the When rules that run in between your "Mapping Begin" event and your Exception is MyServicePage. If you want to broadcast a signal from the Service Activity to be interrogated by the Mapping conditions, then set properties on MyServicePage (which is the Primary Page of the Service Activity) and implement When rules that test for those properties.
This has the added benefit of allowing you to configure different response mappings for different error conditions - if you wanted to. Currently, you'd have to respond with a generic error regardless of how the service activity failed.