Clean up and unify exercises

exercises/13_error_handling/errors3.rs

@@ -1,11 +1,6 @@
-// errors3.rs
-//
 // This is a program that is trying to use a completed version of the
 // `total_cost` function from the previous exercise. It's not working though!
 // Why not? What should we do to fix it?
-//
-// Execute `rustlings hint errors3` or use the `hint` watch subcommand for a
-// hint.
 
 use std::num::ParseIntError;