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/result1.rs

@@ -1,6 +1,8 @@
 // result1.rs
 // Make this test pass! Scroll down for hints :)
 
+// I AM NOT DONE
+
 #[derive(PartialEq, Debug)]
 struct PositiveNonzeroInteger(u64);