Reports WebSocket endpoint declaration errors.

According to the JSR 356 specification, a WebSocket endpoint must have exactly one @OnMessage method for each message type: text, message, and pong

An endpoint class annotated with javax.websocket.server.ServerEndpoint can have exactly one method with each of the lifecycle annotations:

• @OnOpen
• @OnClose
• @OnError

An endpoint class extended from javax.websocket.Endpoint must implement the corresponding lifecycle methods instead of using annotations.