In Rust 1.40, [the #[non_exhaustive] attribute was introduced|https://blog.rust-lang.org/2019/12/19/Rust-1.40.0.html#[non_exhaustive]-structs,-enums,-and-variants]. This attribute allows us to mark various structs and enums as needing a wildcard case when being constructed or pattern matched, which means that adding new cases to the ErrorKind enum or the FindOptions struct no longer break source compatibility.

We should mark all appropriate structs and enums with this attribute.