[IMP] point_of_sale: more informative error

The pos loading crashes in the following scenario.
1. start selling a product (order line the localstorage)
2. archive or delete the product in backend
3. reload the pos session

The POS is interrupted with not much information.
Give an error a bit more meaningful.

addons/point_of_sale/static/src/js/models.js

@@ -1081,7 +1081,8 @@ exports.Orderline = Backbone.Model.extend({
     init_from_JSON: function(json) {
         this.product = this.pos.db.get_product_by_id(json.product_id);
         if (!this.product) {
-            console.error('ERROR: attempting to recover product not available in the point of sale');
+            console.error('ERROR: attempting to recover product ID', json.product_id,
+                'not available in the point of sale. Correct the product or clean the browser cache.');
         }
         this.price = json.price_unit;
         this.set_discount(json.discount);