Show appropriate error when device doesn’t exists on unregisterDevice

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 }
```

Start at the unregisterDevice entry point and reproduce the request for a device that does not exist. Trace how the current 403 error is surfaced from the WebSocket adapter, then verify that the missing-device case returns an appropriate error instead of the empty message shown in the report.

Written by the indexing model from the issue text.