新增于: v0.1.90
一旦套接字完全关闭就触发。
参数 hadError 是布尔值，表示套接字是否由于传输错误而关闭。
Added in: v0.1.90
hadError <boolean> true if the socket had a transmission error.
Emitted once the socket is fully closed. The argument hadError is a boolean
which says if the socket was closed due to a transmission error.