Improve Exception Handling

Moved the mapException() method to PostFinanceCheckout class and also use it in ::createPayment() to map exceptions thrown by ::createTransactionNoUserInteraction().

Tested this with an invalid email and invalid API credentials. In both cases, the user is redirected to the order information page and a generic message is displayed.

The specific exception message is recorded in the logs.

Thanks @coffeemakr

Automatically closed - issue fixed for 2 weeks with no activity.