chore: checkpoint before Python removal

vendor/tower/src/util/optional/error.rs

@@ -0,0 +1,21 @@
+use std::{error, fmt};
+
+/// Error returned if the inner [`Service`] has not been set.
+///
+/// [`Service`]: crate::Service
+#[derive(Debug)]
+pub struct None(());
+
+impl None {
+    pub(crate) fn new() -> None {
+        None(())
+    }
+}
+
+impl fmt::Display for None {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "None")
+    }
+}
+
+impl error::Error for None {}