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/errors3.rs

@@ -3,6 +3,8 @@
 // `total_cost` function from the previous exercise. It's not working though!
 // Why not? What should we do to fix it? Scroll for hints!
 
+// I AM NOT DONE
+
 use std::num::ParseIntError;
 
 fn main() {