fix: ignore errors when forcefully closing the socket (#601)

In order to catch the following errors:

```
events.js:288
      throw er; // Unhandled 'error' event
      ^

Error: write EPIPE
    at afterWriteDispatched (internal/stream_base_commons.js:154:25)
    at writeGeneric (internal/stream_base_commons.js:145:3)
    at Socket._writeGeneric (net.js:780:11)
    at Socket._write (net.js:792:8)
    at doWrite (_stream_writable.js:441:12)
    at writeOrBuffer (_stream_writable.js:425:5)
    at Socket.Writable.write (_stream_writable.js:316:11)
    at abortConnection (<myproject>/node_modules/engine.io/lib/server.js:506:12)
    at <myproject>/node_modules/engine.io/lib/server.js:353:7
    at Server.verify (<myproject>/node_modules/engine.io/lib/server.js:158:14)
    at Server.handleUpgrade (<myproject>/node_modules/engine.io/lib/server.js:351:8)
```

Closes https://github.com/socketio/engine.io/issues/596, https://github.com/socketio/engine.io/pull/598