Fix exit code

[va-an]

Fix for #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.

[tvpeter]

@va-an, could you please rebase and confirm if the issue still exists; otherwise close the PR. Thank you for your efforts.

[va-an]

@va-an, could you please rebase and confirm if the issue still exists; otherwise close the PR. Thank you for your efforts.

I've replied in #170.
Let's discuss there.

[va-an]

@tvpeter, rebased to master.
I composed a command as close as possible to the one in the original issue in order to reproduce the behavior.

From master - exit code 0:

-> % cargo run --features electrum -- \
    --network signet \
    wallet \
    --int-descriptor "" \
    --client-type electrum \
    --database-type sqlite \
    --url some \
    new_address
[2025-04-18T14:31:56Z ERROR bdk_cli] Generic error: An external descriptor is required.
-> % echo $?
0

With fix - exit code 1:

-> % cargo run --features electrum -- \
    --network signet \
    wallet \
    --int-descriptor "" \
    --client-type electrum \
    --database-type sqlite \
    --url some \
    new_address
[2025-04-18T14:31:19Z ERROR bdk_cli] Generic error: An external descriptor is required.
-> % echo $?
1

[va-an]

Please let me know what I can do to make this PR ready to merge.
In the meantime, I'll continue studying the bdk source code to be able to make more significant changes in the future.

[va-an]

Made a force-push to sign my commit.

[notmandatory]

ACK 04c7f88

[tvpeter]

@va-an please rebase.