Clean up and unify exercises

exercises/13_error_handling/errors2.rs

@@ -1,5 +1,3 @@
-// errors2.rs
-//
 // Say we're writing a game where you can buy items with tokens. All items cost
 // 5 tokens, and whenever you purchase items there is a processing fee of 1
 // token. A player of the game will type in how many items they want to buy, and
@@ -15,9 +13,6 @@
 //
 // There are at least two ways to implement this that are both correct-- but one
 // is a lot shorter!
-//
-// Execute `rustlings hint errors2` or use the `hint` watch subcommand for a
-// hint.
 
 use std::num::ParseIntError;