To simplify even more, you don't
actually need `Display` or `Error` impls. I use Error enums frequently without display or Error impls. Though, with better backtrace support coming it'll be handy to have Error and the Backtrace related APIs implemented.
(Sidenote, I don't use `impl Error` because I never use `Box<dyn Error>`. Which isn't to say that's correct, just to say that I've never had the need to implement it)
