Merge bitcoindevkit/bdk-cli#324: fix(combine_psbt): Propagate Psbt::combine errors

Merge bitcoindevkit/bdk-cli#324: fix(combine_psbt): Propagate Psbt::combine errors

682e4148b1994c452dd8619f5a5c43a8ef135e2d fix(combine_psbt): Propagate Psbt::combine errors (Vihiga Tyonum)

Pull request description:

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  ### Description

  `wallet ... combine_psbt` discarded the result of every `Psbt::combine` call, so two BIP174-incompatible PSBTs (different unsigned transactions) produced exit code 0 and a PSBT that was silently missing the other PSBT's inputs and signatures.

  This PR propagates the error with `?`. `BDKCliError` already has a `#[from]` impl for `bitcoin::psbt::Error`, so no new error variant was needed.

  Fixes #323

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  ## Changelog notice

  - Fixed `combine_psbt` to fail when PSBTs cannot be combined instead of silently dropping them

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  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

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

ACKs for top commit:
  vadim-anfv:
    tACK 682e4148b1994c452dd8619f5a5c43a8ef135e2d

Tree-SHA512: d630840f095bc0d48688a9b95140ceaedfb704e8c8ba0ed0fbe5c12c8d5181ba604ed47b2a8b28cae23cac1100d7285524a9c527ab9684c624c327b39cd45eba