Rework auto retry send errors

Rework auto retry send errors

Prior to this, we returned PaymentSendFailure from auto retry send payment
methods. This implied that we might return a PartialFailure from them, which
has never been the case. So it makes sense to rework the errors to be a better
fit for the methods.

We're taking error handling in a totally different direction now to make it
more asynchronous, see send_payment_internal for more information.