Provide a Checkout pane for processing payment when in the shortcut flow.

When in the "shortcut" flow, we cannot rely on the core PaymentProcess pane for capturing/authorizing payment since it depends on the PaymentInformation which is not present in the custom Paypal Checkout checkout_flow the module provides.

We need to provide a custom CheckoutPane that'll redirect to the cart page in case of a PaymentGateway exception.