Improved debugging for wally_psbt_sign_bip32

[Sjors]

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.

Sjors/libwally-swift#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.