Validation step tweaks, remove remaining references to old version

inventory-server/src/routes/import.js

@@ -1394,7 +1394,7 @@ router.get('/check-upc-and-generate-sku', async (req, res) => {
     
     if (upcCheck.length > 0) {
       return res.status(409).json({ 
-        error: 'UPC already exists', 
+        error: 'A product with this UPC already exists', 
         existingProductId: upcCheck[0].pid,
         existingItemNumber: upcCheck[0].itemnumber
       });