Show appropriate error when device doesn’t exists on unregisterDevice

[netoax]

The error is currently being threw as follows:

{ Error: 403 - ""
    at Client.onMessage (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/@cesarbr/knot-cloud-websocket/dist/main.js:1392:23)
    at WebSocket.onMessage (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/ws/lib/event-target.js:120:16)
    at WebSocket.emit (events.js:198:13)
    at Receiver.receiverOnMessage (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/ws/lib/websocket.js:789:20)
    at Receiver.emit (events.js:198:13)
    at Receiver.dataMessage (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/ws/lib/receiver.js:422:14)
    at Receiver.getData (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/ws/lib/receiver.js:352:17)
    at Receiver.startLoop (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/ws/lib/receiver.js:138:22)
    at Receiver._write (/Users/jneto/workspace/knot/cloud/knot-cloud/node_modules/ws/lib/receiver.js:74:10)
    at doWrite (_stream_writable.js:415:12)
  frame:
   '{"type":"error","data":{"code":500,"message":"403 - \\"\\""}}',
  code: 500 }