Clean up and unify exercises

exercises/13_error_handling/errors6.rs

@@ -1,13 +1,8 @@
-// errors6.rs
-//
 // Using catch-all error types like `Box<dyn error::Error>` isn't recommended
 // for library code, where callers might want to make decisions based on the
 // error content, instead of printing it out or propagating it further. Here, we
 // define a custom error type to make it possible for callers to decide what to
 // do next when our function returns an error.
-//
-// Execute `rustlings hint errors6` or use the `hint` watch subcommand for a
-// hint.
 
 use std::num::ParseIntError;