fix: make warning for previously existing SO an alert

(cherry picked from commit 24dcd64c16)

erpnext/selling/doctype/sales_order/sales_order.py

@@ -249,7 +249,8 @@ class SalesOrder(SellingController):
 					frappe.msgprint(
 						_("Warning: Sales Order {0} already exists against Customer's Purchase Order {1}").format(
 							frappe.bold(so[0][0]), frappe.bold(self.po_no)
-						)
+						),
+						alert=True,
 					)
 				else:
 					frappe.throw(