payjoin-cli: surface bitcoin RPC error instead of generic message

[abhinavraajput]

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Fixes #1258

This PR improves error handling in payjoin-cli by propagating the
underlying Bitcoin RPC error message instead of replacing it with
a generic error. This allows users to see actionable RPC errors
such as unloaded wallets.

AI Assistance Disclosure:
I used ChatGPT to understand the codebase and reason about the fix.
The implementation was written and reviewed manually.

[benalleng]

Always nice to see errors getting propagated further but I am skeptical if this is the relevant line in the case of this issue. Notice that the "Caused by:" return states a 500 error where this should never be able to deviate from the return string in the diff.

[abhinavraajput]

@benalleng Thanks for the review!

That makes sense — I agree this change may not be the layer where the
500 status is being introduced for this issue. I’ll take another look
at where the Bitcoin RPC error is being mapped to an HTTP 500 response
and try to address the propagation closer to that boundary instead.

[zealsham]

@abhinavraajput i think the work required to fix this starts from looking at the upstream code and work down from there

[abhinavraajput]

@zealsham Thanks, that’s helpful.

I’ll take a look at the upstream error handling (e.g. where the
Bitcoin RPC / HTTP response is constructed) and trace how the 500
status is introduced before it reaches the CLI. If it turns out
this is best fixed upstream, I’m happy to close this PR and follow
up in the appropriate repository instead.