Allow wrapping of error metadata.

[abourget]

This PR enables wrapping (errors.As) of slack responses which contain rich error messages, e.g., indicating which blocks of position in the submitted JSON the error was found.

Solves #939

[abourget]

Then users can do something like:

func enrichError(err error) error {
	var fullErr slack.SlackErrorResponse
	if errors.As(err, &fullErr) {
		err = fmt.Errorf("%s: %s", err.Error(), strings.Join(fullErr.ResponseMetadata.Messages, ", "))
	}
	return err
}

and simply decorate their err.

[kanata2]

LGTM. Thanks :)