[FIX] website_sale: fix redirect on wrong coupon code

This commits fixes the case where you click on
edit address in eCommerce payment flow and you
type a wrong coupon code.

Without this commit, you are redirected at
/shop/payment instead of /shop/checkout

OPW-2290188

closes odoo/odoo#54559

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>