fix: tweak detail and code of error message

program/services.py

@@ -825,7 +825,9 @@ def make_virtual_timeslot_entry(*, gap_start: datetime, gap_end: datetime) -> Vi
             "start": gap_start.strftime("%Y-%m-%dT%H:%M:%S%z"),
         }
     else:
-        raise NotFound("Radio settings with fallbacks not found.")
+        raise NotFound(
+            detail=_("Radio settings with fallbacks not found."), code="radio_settings-not_found"
+        )
 
 
 def get_timerange_timeslot_entries(