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/errors1.rs

@@ -6,6 +6,8 @@
 // this function to have.
 // Scroll down for hints!!!
 
+// I AM NOT DONE
+
 pub fn generate_nametag_text(name: String) -> Option<String> {
     if name.len() > 0 {
         Some(format!("Hi! My name is {}", name))