Implement Display trait on CoseError enum.

[scouten-adobe]

Resolves #44.

[daviddrysdale]

Looks good, only minor things to mention.

Also, could you add something like the following to CHANGELOG.md please:

## 0.3.1 - TBD

- Implement `Display` for `CoseError`.

[daviddrysdale]

This is failing a CI check because we've got a local convention that things that panic! need a comment of the form "safe: ..." on the same line to document why it's OK to panic. (Unfortunately the check is very primitive, so it doesn't notice that this is #[cfg(test)] code.)

[daviddrysdale]

Nope, it's an even dumber check than that :-( (./scripts/check_format.sh).

Let me see if I can make it less pesky…

[scouten-adobe]

Oh, interesting challenge. If the panic! spreads across multiple lines, cargo fmt wants to put the comment on a line of its own.

Might be simpler for me to rewrite this as an assert!(false...).

[daviddrysdale]

Yeah, cargo fmt and check_format.sh fight each other.

Shifting to assert should work though … the check is just for panic and the red things on https://tinyurl.com/rust-transform.

[daviddrysdale]

Thanks!