Add warning about potentially-missing no-bundler feature

src/workerHelpers.js

@@ -51,7 +51,10 @@ waitForMsgType(self, 'wasm_bindgen_worker_init').then(async data => {
   // OTOH, even though it can't be inlined, it should be still reasonably
   // cheap since the requested file is already in cache (it was loaded by
   // the main thread).
-  const pkg = await import('../../..');
+  const pkg = await import('../../..').catch(e => {
+    console.warn("Are you using a bundler? If not, you should use the `no-bundler` feature to fix this error, as explained in the wasm-bindgen-rayon readme.");
+    throw new Error(e);
+  });
   await pkg.default(data.module, data.memory);
   postMessage({ type: 'wasm_bindgen_worker_ready' });
   pkg.wbg_rayon_start_worker(data.receiver);