Do we want special support for error "hints", or long error messages?

See #578 (closed): I want to display a much longer message on a given failure type when its facing the user. But how to do it? The desired message is significantly larger than a typical developer-facing error message.

Some possibilities include:

1. Introduce an ErrorHint trait, or something, plus machinery to recursively inspect all our errors. (IMO this would be a pain without macro support, since we can't recurse on Error::source and probe whether each Error implements ErrorHint.)
2. Mess around with the alternative formatter code, and display our errors with {:#} when they are going to a human. (This might require some fooling around with thiserror, and might not even work.)
3. Dispatch based on the ErrorKind value, and require that any ErrorKind with a hint should only appear when that hint is appropriate.
4. Add a "hint" field to ErrorKind, with type Option. (This would make our ErrorKind construction much nastier.
5. Bite the bullet, and just declare that our errors sometimes have long messages.