Update Exercises Directory Names to Reflect Order

exercises/13_error_handling/errors4.rs

@@ -0,0 +1,32 @@
+// errors4.rs
+//
+// Execute `rustlings hint errors4` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+#[derive(PartialEq, Debug)]
+struct PositiveNonzeroInteger(u64);
+
+#[derive(PartialEq, Debug)]
+enum CreationError {
+    Negative,
+    Zero,
+}
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
+        // Hmm... Why is this always returning an Ok value?
+        Ok(PositiveNonzeroInteger(value as u64))
+    }
+}
+
+#[test]
+fn test_creation() {
+    assert!(PositiveNonzeroInteger::new(10).is_ok());
+    assert_eq!(
+        Err(CreationError::Negative),
+        PositiveNonzeroInteger::new(-10)
+    );
+    assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
+}