radicle: mark `CheckoutError` as `non_exhaustive`

Recently, this error enum was changed. To ensure it does not cause crate version
changes, in the future, mark it as `non_exhaustive`.

crates/radicle/src/rad.rs

@@ -225,6 +225,7 @@ where
 }
 
 #[derive(Error, Debug)]
+#[non_exhaustive]
 pub enum CheckoutError {
     #[error("failed to fetch to working copy: {0}")]
     FetchIo(#[source] std::io::Error),