Wrapped multi-errors should implement Unwrap() []error

[abhinav]

errtrace errors only implement Unwrap() error right now.
This hides the actual list of errors for wrapped multi-errors.
We need to implement Unwrap() []error if the error we're wrapping is a multi-error.

Alternatively, we can always implement only Unwrap() []error, and things should still just work.

[abhinav]

This hides the actual list of errors for wrapped multi-errors.

Actually, this isn't quite right. Even with just Unwrap() error, we're exposing the underlying list on the "next" layer, so it's not entirely inaccessible. But it's still going to be annoying. Although I'm not entirely certain that implementing Unwrap() []error is the right choice there, because transparently skipping the actual wrapped error means fmt.Errorf("information: %w %w", a, b) will unpack to a and b, ignoring information entirely.

Separately, maybe we also need an errtrace.Unwrap that's a drop-in replacement for errors.Unwrap, and all it does is transparently unwrap the top-level errtrace error—if any. That can be a separate discussion.