fix: return psbt spend utxo errors

[reez]

Description

I think it would be good for Psbt::spend_utxo to return typed PsbtErrors instead of potentially panicking when rust-bitcoin reports missing or outofbounds spend UTXOs.

Notes to the reviewers

Documentation

• bdk_wallet
• bitcoin
  • Psbt::spend_utxo
  • SignError
• uniffi
• Other:

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[ItoroD]

ACK 23bb9a9

Learning a lot from these error handling PRs.