Expose per-product forecast errors

docs/modules/erp/lager/processes/main/erp.lager.stock_lot_forecast_refresh.md

@@ -82,7 +82,8 @@ Nicht erforderlich. Die Produktmenge ist klein; die Verarbeitung erfolgt determi
   "total_products": 0,
   "processed_products": 0,
   "failed_products": 0,
-  "failed_product_ids": []
+  "failed_product_ids": [],
+  "errors": [{"product_id": 0, "message": "..."}]
 }
 ```