Mark public enums non-exhaustive

[tgross35]

Fail, Occur, and ParsingStyle could all have legitimate reasons to gain options in the future, so for 0.3 we should mark them #[non_exhaustive].

It may also be worth doing some renaming to make things more consistent with the rest of the ecosystem:

• Fail -> Error
• Occur::Req -> Required
• Occur::Multi -> Occur::ZeroOrMore