Merge bitcoindevkit/bdk-cli#173: Fix exit code

Merge bitcoindevkit/bdk-cli#173: Fix exit code

304729b173a00a908d7c0854fbe1fd6dd541d29e fix: use exit code 1 in case of error (Vadim Anufriev)

Pull request description:

  Fix for https://github.com/bitcoindevkit/bdk-cli/issues/170.

  I think this is the simplest way to do it.
  It's also possible to change the main signature to return `Result`, which should provide similar behavior, but this option isn't compatible with the `#[maybe_async]` macro, so I decided not to complicate things.

  ### 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
  * [ ] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

Top commit has no ACKs.

Tree-SHA512: 52ce4292c3a8347bdb6ec975bb33a728c8eb2a1cdc90647a2834fc3428106a4aef1c3cf6b526905e5a36ea2734759338b5dfd4d3a55778507fc59893f585cf7d