When a user starts a payment in the Marketplace, webpay will attempt to find a payment provider to use based on which accounts the app has configured and where the user is located (with regards to MNC/MCC). If it can not find a valid payment provider, it will raise an exception. Presently I believe this is unhandled and will not be displayed meaningfully to the user. We need to handle this case and inform the user that they are unable to complete the transaction.

Usecases: 1. User on Telcel SIM tries to buy an app which has Bango set up but no Boku. 2. User on Movistar SIM tries to buy an app which has Boku set up but no Bango. 3. User on Telcel SIM tries to buy an app which has BOTH bango and boku but has a price tier that is not supported by boku.

right now I think this shows a TRANS_NOT_FOUND code and a generic error message so maybe we can improve that.

Postponing some work while I focus on Mozilla's new payments service