Expose `Ed25519Error` type externally or add mapping of it to `RecryptErr` type.

[ernieturner]

We recently changed the library to expose the RecryptErr type which was accidentally not being exposed out of the library. Now I noticed that we have the same problem with the Ed25519Error type. From what I can tell it's only ever used as part of the SigningKeypair methods, so maybe the better path forward here would be to get rid of it in favor of the RecryptErr type.