error/success handling (user feedback)

src/lib.rs

@@ -41,6 +41,11 @@ pub fn load_additional(game_data: String) -> String {
 #[wasm_bindgen]
 pub fn merge() -> String {
     let state = STATE.lock().unwrap();
+
+    if state.main.is_none() || state.additional.is_none() {
+        return "".to_string();
+    }
+
     let mut main = state.main.clone().unwrap();
     let additional = state.additional.clone().unwrap();
     main.merge(additional);