Update WebSocket.cpp

bufix: when connecting fail, libwebsocket_client_connect() will return NULL, and onSocketCallback() will not be called, so we need trigger WS_MSG_TO_UITHREAD_ERROR here.
Merge commit 'refs/pull/5232/head' of git://github.com/cocos2d/cocos2d-x into merge5232
closed #4101: No callback is invoked when websocket connection fails
Merge pull request #5415 from dumganhar/merge5232

closed #4101: No callback is invoked when websocket connection fails