[ServiceWorker] Refine error message and fix lint warning

src/message.ts

@@ -29,6 +29,7 @@ type RequestKind =
   | "keepAlive"
   | "heartbeat";
 
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
 type ResponseKind =
   | "return"
   | "throw"

src/service_worker.ts

@@ -1,5 +1,5 @@
 import * as tvmjs from "tvmjs";
 import { AppConfig, ChatOptions, MLCEngineConfig, ModelRecord } from "./config";
 import { ReloadParams, WorkerRequest, WorkerResponse } from "./message";
 import { MLCEngineInterface, InitProgressReport } from "./types";
 import {
@@ -40,7 +40,7 @@
   >();
   private initReuqestUuid?: string;
 
   constructor(engine: MLCEngineInterface, verbose = false) {
     if (!self || !("addEventListener" in self)) {
       throw new Error(
         "ServiceWorkerGlobalScope is not defined. ServiceWorkerMLCEngineHandler must be created in service worker script.",
@@ -188,13 +188,21 @@
   engineConfig?: MLCEngineConfig,
 ): Promise<ServiceWorkerMLCEngine> {
   if (!("serviceWorker" in navigator)) {
-    throw new Error("Service worker API is not available");
+    throw new Error(
+      "Service worker API is not available in your browser. Please ensure that your browser supports service workers and that you are using a secure context (HTTPS). " +
+        "Check the browser compatibility and ensure that service workers are not disabled in your browser settings.",
+    );
   }
-  const registration = await (navigator.serviceWorker as ServiceWorkerContainer)
-    .ready;
-  const serviceWorkerMLCEngine = new ServiceWorkerMLCEngine(
-    registration.active!,
-  );
+  const serviceWorkerAPI = navigator.serviceWorker as ServiceWorkerContainer;
+  const registration = await serviceWorkerAPI.ready;
+  const serviceWorker = registration.active || serviceWorkerAPI.controller;
+  if (!serviceWorker) {
+    throw new Error(
+      "Service worker failed to initialize. This could be due to a failure in the service worker registration process or because the service worker is not active. " +
+        "Please refresh the page to retry initializing the service worker.",
+    );
+  }
+  const serviceWorkerMLCEngine = new ServiceWorkerMLCEngine(serviceWorker);
   serviceWorkerMLCEngine.setInitProgressCallback(
     engineConfig?.initProgressCallback,
   );
@@ -212,7 +220,7 @@
 export class ServiceWorkerMLCEngine extends WebWorkerMLCEngine {
   missedHeatbeat = 0;
 
   constructor(worker: IServiceWorker, keepAliveMs = 10000, verbose = false) {
     if (!("serviceWorker" in navigator)) {
       throw new Error("Service worker API is not available");
     }