[IMP] sale: improve error message when trying to delete confirmed so lines

Closes #18521 .

addons/sale/models/sale.py

@@ -1056,7 +1056,7 @@ class SaleOrderLine(models.Model):
     @api.multi
     def unlink(self):
         if self.filtered(lambda x: x.state in ('sale', 'done')):
-            raise UserError(_('You can not remove a sales order line.\nDiscard changes and try setting the quantity to 0.'))
+            raise UserError(_('You can not remove an order line once the sales order is confirmed.\nYou should rather set the quantity to 0.'))
         return super(SaleOrderLine, self).unlink()
 
     @api.multi