fix(errorsn): Try harder to confine the user. (rust-lang#388)

exercises/error_handling/errorsn.rs

@@ -2,8 +2,10 @@
 // This is a bigger error exercise than the previous ones!
 // You can do it! :)
 //
-// Edit the `read_and_validate` function so that it compiles and
-// passes the tests... so many things could go wrong!
+// Edit the `read_and_validate` function ONLY. Don't create any Errors
+// that do not already exist.
+//
+// So many things could go wrong!
 //
 // - Reading from stdin could produce an io::Error
 // - Parsing the input could produce a num::ParseIntError
@@ -28,6 +30,10 @@ fn read_and_validate(b: &mut dyn io::BufRead) -> Result<PositiveNonzeroInteger,
     Ok(answer)
 }
 
+//
+// Nothing below this needs to be modified
+//
+
 // This is a test helper function that turns a &str into a BufReader.
 fn test_with_str(s: &str) -> Result<PositiveNonzeroInteger, Box<dyn error::Error>> {
     let mut b = io::BufReader::new(s.as_bytes());