Add error message to device registry helper (#118676)

homeassistant/helpers/device_registry.py

@@ -820,7 +820,9 @@ class DeviceRegistry(BaseRegistry[dict[str, list[dict[str, Any]]]]):
             )
 
         if merge_identifiers is not UNDEFINED and new_identifiers is not UNDEFINED:
-            raise HomeAssistantError
+            raise HomeAssistantError(
+                "Cannot define both merge_identifiers and new_identifiers"
+            )
 
         if isinstance(disabled_by, str) and not isinstance(
             disabled_by, DeviceEntryDisabler