Start an error handling section and move the result exercise there

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

standard_library_types/result1.rs

@@ -1,42 +0,0 @@
-// 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 :)