fix(combine_psbt): Propagate Psbt::combine errors

fix(combine_psbt): Propagate Psbt::combine errors

The fold merging the input PSBTs discarded each combine result with
`let _ = acc.combine(x)`. Per BIP174, Psbt::combine returns Err whenever
the two PSBTs do not describe the same unsigned transaction, so
incompatible inputs left the accumulator untouched.

This fix propagates the error with `?`. BDKCliError already converts from
bitcoin::psbt::Error, so the command now exits non-zero.

Fixes #323