Improved debugging for wally_psbt_sign_bip32

I'm trying to leverage the new `wally_psbt_sign_bip32` function, but having a hard time debugging things, since it only returns `WALLY_EINVAL`.

https://github.com/Sjors/libwally-swift/pull/85

Different error codes for different failure modes would make this easier. Alternatively, having more helper methods to sanity check inputs could help debugging too. E.g. I ended up calling `wally_psbt_get_id` (which succeeds) since it calls some common code with `wally_psbt_sign_bip32` to narrow down the failure.