Map finalize_proposal error appropriately in v1/v2 state machines

[spacebear21]

V1/V2 could both call this abstraction (perhaps call it psbt_ctx.finalize_proposal) and map that abstraction's error as appropriate for the particular state machine or to a session.

Originally posted by @DanGould in #905 (comment)

See the full discussion quoted above for more context.

[DanGould]

Closed by #938