If you are debugging I'd suggest dumping the entire session to the page and you can look for what is erroring:
If you are thinking of adding client-facing errors, then you need to be a little careful. Credit card companies suggest not including any identifying information when cards fail. If you include a message like "Expiration Month is wrong" then if someone has a stolen card number they can just try different months.
If you are looking for information on failed transactions, I'd suggest emailing it to yourself and not displaying it to the client. Maybe just ask the client to call on the failure page and email you the session dump so you can look into it if they call.