Audit payjoin-ffi's `ImplementationError` `From` implementation

[spacebear21]

We need to make sure our error conversions are sane so that arbitrary types don't get converted into unrelated errors accidentally.

In payjoin-ffi, currently any arbitrary String can be converted to an ImplementationError. Maybe we should just use anyhow::Error?

See discussion in LtbLightning/payjoin-ffi#71 (comment)

[spacebear21]

Related issue: #751

[arminsabouri]

@spacebear21 is should this be 1.0 milestone issue?

[spacebear21]

Hmm, it only touches payjoin-ffi and downstream so technically we could release a payjoin-1.0 before this is done. But for any dart/python 1.0 release I think this is necessary.

[arminsabouri]

cc @chavic

[arminsabouri]

TODO: assign chavic to this ticket