Custom error page for checkout

In case there's a Server Side error on a checkout page, you can redirect users to a custom error page related to the checkout.

The default API calls contain a parameter (third method parameter) ownErrorHandler: 1 which enables you to use a custom error callback (sixth method parameter) which can do whatever you want to, including using app.getRouter().push(<route>). For example:

	status: status,
	ownErrorHandler: 1,
     (cart) => {
	Checkout.addCheckoutEvent('payment', null, cart) // Credit Card
     (error) => {
	app.getLoader('context').notifyUser(error.message, 'error')
	     type: 'CartApi.Cart.update.error',
	     error: error,