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/option1.rs

@@ -4,6 +4,8 @@
 // on `None`. Handle this in a more graceful way than calling `unwrap`!
 // Scroll down for hints :)
 
+// I AM NOT DONE
+
 pub fn pop_too_much() -> bool {
     let mut list = vec![3];