Add errors to exercises that compile without user changes

exercises/move_semantics/move_semantics4.rs

@@ -16,7 +16,8 @@ fn main() {
 
 }
 
-fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
+// `fill_vec()` no longer take `vec: Vec<i32>` as argument
+fn fill_vec() -> Vec<i32> {
     let mut vec = vec;
 
     vec.push(22);