Start an error handling section and move the result exercise there

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

README.md

@@ -91,14 +91,14 @@ Note that the exercises in this section may look similar to each other but they
 - ["modules1.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Amod+sausage_factory+%7B%0A++++fn+make_sausage%28%29+%7B%0A++++++++println%21%28%22sausage%21%22%29%3B%0A++++%7D%0A%7D%0A%0Afn+main%28%29+%7B%0A++++sausage_factory%3A%3Amake_sausage%28%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Everything+is+private+in+Rust+by+default--+but+there%27s+a+keyword+we+can+use%0A%2F%2F+to+make+something+public%21+The+compiler+error+should+point+to+the+thing+that%0A%2F%2F+needs+to+be+public.%0A)
 - ["modules2.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Amod+us_presidential_frontrunners+%7B%0A++++use+self%3A%3Ademocrats%3A%3AHILLARY_CLINTON+as+democrat%3B%0A++++use+self%3A%3Arepublicans%3A%3ADONALD_TRUMP+as+republican%3B%0A%0A++++mod+democrats+%7B%0A++++++++pub+const+HILLARY_CLINTON%3A+%26%27static+str+%3D+%22Hillary+Clinton%22%3B%0A++++++++pub+const+BERNIE_SANDERS%3A+%26%27static+str+%3D+%22Bernie+Sanders%22%3B%0A++++%7D%0A%0A++++mod+republicans+%7B%0A++++++++pub+const+DONALD_TRUMP%3A+%26%27static+str+%3D+%22Donald+Trump%22%3B%0A++++++++pub+const+JEB_BUSH%3A+%26%27static+str+%3D+%22Jeb+Bush%22%3B%0A++++%7D%0A%7D%0A%0Afn+main%28%29+%7B%0A++++println%21%28%22candidates%3A+%7B%7D+and+%7B%7D%22%2C%0A+++++++++++++us_presidential_frontrunners%3A%3Ademocrat%2C%0A+++++++++++++us_presidential_frontrunners%3A%3Arepublican%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+The+us_presidential_frontrunners+module+is+trying+to+present+an+external%0A%2F%2F+interface+%28the+%60democrat%60+and+%60republican%60+constants%29+that+is+different+than%0A%2F%2F+its+internal+structure+%28the+%60democrats%60+and+%60republicans%60+modules+and%0A%2F%2F+associated+constants%29.+It%27s+almost+there+except+for+one+keyword+missing+for%0A%2F%2F+each+constant.%0A%2F%2F+One+more+hint%3A+I+wish+the+compiler+error%2C+instead+of+saying+%22unresolved+name%0A%2F%2F+%60us_presidential_frontrunners%3A%3Ademocrat%60%22%2C+could+say++%22constant%0A%2F%2F+%60us_presidential_frontrunners%3A%3Ademocrat%60+is+private%22%21)
 
-### Standard library types
-
-#### `Result`
+### Error Handling
 
 The [Error Handling](https://doc.rust-lang.org/stable/book/error-handling.html) and [Generics](https://doc.rust-lang.org/stable/book/generics.html) sections are relevant.
 
 - ["result1.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+this+test+pass%21+Scroll+down+for+hints+%3A%29%0A%0A%23%5Bderive%28PartialEq%2CDebug%29%5D%0Astruct+PositiveNonzeroInteger%28u64%29%3B%0A%0A%23%5Bderive%28PartialEq%2CDebug%29%5D%0Aenum+CreationError+%7B%0A++++Negative%2C%0A++++Zero%2C%0A%7D%0A%0Aimpl+PositiveNonzeroInteger+%7B%0A++++fn+new%28value%3A+i64%29+-%3E+Result%3CPositiveNonzeroInteger%2C+CreationError%3E+%7B%0A++++++++Ok%28PositiveNonzeroInteger%28value+as+u64%29%29%0A++++%7D%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_creation%28%29+%7B%0A++++assert%21%28PositiveNonzeroInteger%3A%3Anew%2810%29.is_ok%28%29%29%3B%0A++++assert_eq%21%28Err%28CreationError%3A%3ANegative%29%2C+PositiveNonzeroInteger%3A%3Anew%28-10%29%29%3B%0A++++assert_eq%21%28Err%28CreationError%3A%3AZero%29%2C+PositiveNonzeroInteger%3A%3Anew%280%29%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+%60PositiveNonzeroInteger%3A%3Anew%60+is+always+creating+a+new+instance+and+returning+an+%60Ok%60+result.%0A%2F%2F+It+should+be+doing+some+checking%2C+returning+an+%60Err%60+result+if+those+checks+fail%2C+and+only%0A%2F%2F+returning+an+%60Ok%60+result+if+those+checks+determine+that+everything+is...+okay+%3A%29%0A)
 
+### Standard library types
+
 #### `Arc`
 
 The [Concurrency](https://doc.rust-lang.org/stable/book/concurrency.html) section is relevant.