Fix scope issue in error handler of WebsocketStream

I have been encountering the following error in my logs:
```
TypeError: Cannot call method 'filter' of undefined
    at Stream.<anonymous> (/home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/lib/WebsocketStream.js:46:43)
    at Stream.EventEmitter.emit (events.js:117:20)
    at Stream.stream.emit (/home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/node_modules/stream-serializer/index.js:60:33)
    at Stream.<anonymous> (/home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/node_modules/mux-demux/inject.js:95:22)
    at Stream.stream.write (/home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/node_modules/through/index.js:26:11)
    at /home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/lib/WebsocketStream.js:70:7
    at Array.forEach (native)
    at WebsocketStream.writeActivity (/home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/lib/WebsocketStream.js:69:24)
    at HAProxyManager.<anonymous> (/home/appuser/thalassa-aqueduct/app/node_modules/thalassa-aqueduct/index.js:103:21)
    at HAProxyManager.EventEmitter.emit (events.js:117:20)
npm ERR! weird error 8
npm ERR! not ok code 0
```

Based on the logging available this might be the cause of thalassa-aqueduct restarting unexpectedly.