Start an error handling section and move the result exercise there

I think this is a better categorization than "standard library types".

error_handling/result1.rs

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+// Make this test pass! Scroll down for hints :)
+
+#[derive(PartialEq,Debug)]
+struct PositiveNonzeroInteger(u64);
+
+#[derive(PartialEq,Debug)]
+enum CreationError {
+    Negative,
+    Zero,
+}
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
+        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));
+}
+
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+// `PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
+// It should be doing some checking, returning an `Err` result if those checks fail, and only
+// returning an `Ok` result if those checks determine that everything is... okay :)