feat: emit an Error object upon middleware error

This commit restores the ability to send additional data in the
middleware functions, which was removed during the rewrite to
Typescript ([1]).

The only difference with the previous implementation is that the client
will now emit a "connect_error" (previously, "error") event with an
actual Error object, with both the message and an optional "data"
attribute.

```js
// server-side
io.use((socket, next) => {
  const err = new Error("not authorized");
  err.data = { content: "Please retry later" };
  next(err);
});

// client-side
socket.on("connect_error", err => {
  console.log(err.message); // not authorized
  console.log(err.data.content); // Please retry later
});
```

[1]: https://github.com/socketio/socket.io/commit/a5581a978979ff2c2c53dacdc931e7025c5d6adb