XRBK: use generics in errors instead of requiring casting

[Antikyth]

I'm not sure if this can work or not, but I wonder if it would be possible to do this kinda thing:

pub enum ReadError<Discrim = u32, Other = Infallible> {
    UnrecognisedDiscriminant(Discrim),
    Other(Other),
}

My concern would be how to define in the e.g. Readable trait - should work with associated types I suppose, main unfortunate part is that associated types can't have defaults.