Look into using external types for errors

Uniffi allows us to use types that are defined externally and don't require wrapping directly using a fairly simply workflow.

See https://mozilla.github.io/uniffi-rs/latest/types/remote_ext_types.html#remote-types.

We successfully leveraged this for [LogLevel](https://github.com/bitcoindevkit/bdk-ffi/blob/039d1f02d5a56e569208edd49515bd9c83ae6696/bdk-ffi/src/kyoto.rs#L393C1-L402C2), and I suspect that this might also work for some of our errors! Needs a bit of investigating though.