refactor: rename ERROR to CONNECT_ERROR

The meaning is not modified: this packet type is still used by the
server when the connection to a namespace is refused.

Breaking change: the Socket instance will now emit a "connect_error"
event instead of "error" (which is not a reserved event anymore)

```js
// before
socket.on("error", () => {});

// after
socket.on("connect_error", () => {});
```