Add #[non_exhaustive] to enums that might get new variants. (#673)

This just using them in patterns without a catchall. I left things alone
that seem very unlikely to change (`Value`, `ValueRef`, `DatabaseName`,
etc...). This might help reduce the number of breaking changes we need
(rusqlite is still pre-1.0 so it doesn't really matter that much, but
breaking changes complicate the story around when we can cut releases).

src/error.rs

@@ -10,6 +10,7 @@ use std::str;
 /// Enum listing possible errors from rusqlite.
 #[derive(Debug)]
 #[allow(clippy::enum_variant_names)]
+#[non_exhaustive]
 pub enum Error {
     /// An error from an underlying SQLite call.
     SqliteFailure(ffi::Error, Option<String>),