receive::v2::SessionError variants are not recoverable and must not return JSON

[DanGould]

... because they can't

I've had an inkling this was the case since Jsonifying v2 errors. It also makes me question whether original-psbt-rejected is really the appropriate error code for v1::RequestErrors if any is appropriate at all.

This became even more apparent when implementing #520 since the into_url::Error is a client error that need not be returned to the sender, since they can't recover from it, unlike v1::RequestErrors.