feat: improve watch execution mode

The `watch` command now requires user action to move to the next
exercise.

BREAKING CHANGE: this changes the behavior of `watch`.

exercises/error_handling/errorsn.rs

@@ -15,6 +15,8 @@
 //
 // Scroll down for hints :)
 
+// I AM NOT DONE
+
 use std::error;
 use std::fmt;
 use std::io;