Make primitive_types3 require at least 100 elements

Made the function panic if it's not long enough

exercises/primitive_types/primitive_types3.rs

@@ -14,5 +14,6 @@ fn main() {
         println!("Wow, that's a big array!");
     } else {
         println!("Meh, I eat arrays like that for breakfast.");
+        panic!("Array not big enough, more elements needed")
     }
 }