server: Improve duplicate registration message

jvm/src/main/scala/cs214/webapp/server/web/WebServer.scala

@@ -38,7 +38,7 @@ object WebServer:
 
   /** Registers the given app */
   private def register(app: ServerAppFactory): Unit =
-    require(!appDirectory.contains(app.appInfo.id))
+    require(!appDirectory.contains(app.appInfo.id), f"Already registered: ${app.appInfo.id}")
     appDirectory.put(app.appInfo.id, app)
     println(f"[${app.appInfo.id}] registered")