Localize the custom error message

as documented in:
https://cap.cloud.sap/docs/node.js/app-services#custom-errors

Author: PierreFritsch

bookshop/srv/cat-service.js

@@ -9,7 +9,7 @@ class CatalogService extends cds.ApplicationService { init(){
     const {book,quantity} = req.data
     if (quantity < 1) return req.reject (400,`quantity has to be 1 or more`)
     let {stock} = await SELECT `stock` .from (Books,book)
-    if (quantity > stock) return req.reject (409,`${quantity} exceeds stock for book #${book}`)
+    if (quantity > stock) return req.reject (409, "ORDER_EXCEEDS_STOCK", [quantity, book])
     await UPDATE (Books,book) .with ({ stock: stock -= quantity })
     await this.emit ('OrderedBook', { book, quantity, buyer:req.user.id })
     return { stock }

bookshop/srv/i18n/messages_en.properties

@@ -0,0 +1 @@
+ORDER_EXCEEDS_STOCK={0} exceeds stock for book #{1}