Refactor lpc55_sign error handling

lpc55_sign::Error has become a dumping ground for every function provided in that crate.  While there is some overlap between commands, there's also a bunch that is orthogonal.  Ideally, lpc55_sign::Error would be broken up into a bunch of different error types so users of the error code don't have to deal with 20 variants that can't possible happen on whatever function they are using.