Return error if session closed with a failure

[arminsabouri]

Returning the current state encourages developers to repeat the action that caused the failure. Instead we should error out during session replays.

Blocked by #1129 . Drafting for now. There is a TODO as well that I'd like to address. As of now, IIRC the application never recieves the error that fatally closed the session on a replay. If we include a stringified version of the error that would provide some visibility that previously TerminalFailure(String) provided. Please correct me if my assumption about this is incorrect.

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[coveralls]

Pull Request Test Coverage Report for Build 18199861996

Details

• 60 of 82 (73.17%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 84.636%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin/src/core/error.rs	10	11	90.91%
payjoin/src/core/receive/v2/mod.rs	2	4	50.0%
payjoin/src/core/send/v2/mod.rs	1	4	25.0%
payjoin-cli/src/app/v2/mod.rs	0	4	0.0%
payjoin/src/core/receive/v2/session.rs	42	46	91.3%
payjoin/src/core/send/v2/session.rs	5	13	38.46%

Files with Coverage Reduction	New Missed Lines	%
payjoin-cli/src/app/v2/mod.rs	1	61.45%

Totals
Change from base Build 18176477486:	0.07%
Covered Lines:	8720
Relevant Lines:	10303

---

💛 - Coveralls

[DanGould]

I suppose this implies the presence of a closed event is insufficient to display failure in UI? On its face it doesn't seem like a closed session should throw an error which semantically describes exceptions.

[spacebear21]

the presence of a closed event is insufficient to display failure in UI?

The closed event describes the outcome (e.g. failure) but no details about how it occurred. To do that we need the session history. The current approach is to return the current_state on replay of a Closed event, but @arminsabouri described in OP that it may be a footgun to keep returning the same state. There were two proposed solutions:

1. Re-introduce the TerminalFailure unit variant - this is simpler but also a bit of a hack as it's not a true state in the state machine and can only be reached by replaying the event log.
2. Return an error with the SessionHistory included, so that failure reason can be reconstructed. That's what this PR does

[spacebear21]

#1136 shows the alternative approach of re-introducing a closed variant to the ReceiveSession state machine. In hindsight that does feel cleaner so Approach NACK on this PR in favor of #1136.

[arminsabouri]

superseded by #1132