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/errors2.rs

@@ -16,6 +16,8 @@
 // There are at least two ways to implement this that are both correct-- but
 // one is a lot shorter! Scroll down for hints to both ways.
 
+// I AM NOT DONE
+
 use std::num::ParseIntError;
 
 pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {