This appears to be better?

src/xenia/kernel/app.cc

@@ -30,7 +30,7 @@ X_RESULT XAppManager::DispatchMessageSync(uint32_t app_id, uint32_t message,
                                           uint32_t buffer_length) {
   const auto& it = app_lookup_.find(app_id);
   if (it == app_lookup_.end()) {
-    return X_ERROR_NOT_FOUND;
+    return X_STATUS_UNSUCCESSFUL;
   }
   return it->second->DispatchMessageSync(message, buffer_ptr, buffer_length);
 }