Remove "I AM NOT DONE" and the verify mode and add AppState

exercises/13_error_handling/errors5.rs

@@ -22,8 +22,6 @@
 // Execute `rustlings hint errors5` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
-
 use std::error;
 use std::fmt;
 use std::num::ParseIntError;