Make errAlreadyWroteClose exported

Hello!

I would like to get a mechanism to avoid unnecessary logging of the closing error:
```golang
	defer func() {
		err2 := wsConn.Close(websocket.StatusInternalError, "the sky is falling")
		if err2 != nil {
			logger.WithError(err2).Error("error on close websocket")
		}
	}()
```

Thanks.