message: Better error message in mount engine
diff: @@ -37,6 +37,10 @@ export async function loadEngine(engine: string, mountPoint: string): Promise<En
  // Idempotent router setup, would otherwise be triggered by calling `visit()`
  router.setupRouter();
  + if (!(mountPoint in router._engineInfoByRoute)) {
  + throw new Error(`No engine is mounted at ${mountPoint}`);
  + }
  +
  // Create the engine instance using the engineInfo loaded by calling `setupRouter`
  const instance: EngineInstance = await router._loadEngineInstance(
  router._engineInfoByRoute[mountPoint],
  