add comment to help unknown error message changes

core/tauri/src/window.rs

@@ -1569,6 +1569,8 @@ impl<R: Runtime> Window<R> {
         return Err(Error::InvokeKey);
       }
       None => {
+        // this specific string is tested against in `core/tests/invoke-key`.
+        // if this error changes then that integration test should be updated accordingly.
         let error = "received ipc message without a __TAURI_INVOKE_KEY__";
 
         #[cfg(feature = "tracing")]