Currently we just show a form with missing fields and don't act right. Set a default, or redirect to donatewiki or 'ways to give'.
error example:
Dec 28 17:42:59 payments1005 adyen_gateway: 157052701:157052701.1 Redirecting to fail page for exception in setClientVariables: Configuration key 'payment-provider//class' does not exist.
For Ingenico, we get a form with missing fields and no card logos. Pressing continue yields an error.
https://payments.wikimedia.org/index.php?title=Special:GlobalCollectGateway&appeal=JimmyQuote&ffname=cc-vmad&recurring=&uselang=en&language=en¤cy_code=USD&country=US&amount=1