fix: fire an error event on middleware failure for non-root namespace

[darrachequesne]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

In the following example:

io.use((socket, next) => {
  next(new Error('Auth failed'));
});

// client-side
const socket = io('https://url/custom-namespace');

socket.on('error', (err) => {
  // event was not fired
});

The 'error' event wasn't fired on the custom namespace.

New behaviour

The event is now fired.

Other information (e.g. related issues)

Related:

• Connecting to custom namespace connects to default namespace, too socket.io#3089
• Handshake authorization stuck. socket.io#1934
• Root nsp middleware always called before sub nsp middleware authorization  socket.io#1888