Make Error and GameType enums non-exhaustive

So that it's not a breaking change to add more variants in the future.

src/error.rs

@@ -9,6 +9,7 @@ use nom::error::ErrorKind;
 use nom::Err;
 
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     ParsingIncomplete(MoreDataNeeded),
     // The string is the input that was not parsed.

src/lib.rs

@@ -22,9 +22,8 @@ use function::Function;
 
 type ParsingResult<'a, T> = IResult<&'a str, T, ParsingError<&'a str>>;
 
-// GameType variants must not change order, as their integer values are used as
-// constants in the C API.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[non_exhaustive]
 pub enum GameType {
     Oblivion,
     Skyrim,