Fix abort message when deriving non-unit enum variants

[miguelff]

This PR fixes the abort! message when applying the derive directive to enums containing variants that are non-unit.

I found the previous message confusing due to using double negation:

"#[derive(ValueEnum)] only supports non-unit variants, unless they are skipped"

I believe the message should instead be:

"#[derive(ValueEnum)] only supports non-unit variants if they are skipped"

Or even more explicit, as provided in the patch:

"#[derive(ValueEnum)] only supports unit variants. Non-unit variants must be skipped"

[epage]

Thanks! I like that wording!

This got merged into master which is for v4. You are welcome to backport this to v3-master if you wish.

[miguelff]

You are welcome to backport this to v3-master if you wish.

Done in #4124