Better handling of payment errors

[carnage]

There are a lot of error scenarios in the new payment model, all of which are handled by a single PaymentError event. This should be refactored to allow differentiation between different payment errors and the raising of some errors to the admin for manual intervention.

Some of these changes will be possible in 1.x others may require changes to the event model so will need to be deferred to 2.0.0