Stopping Detection process invokes the error toast

[petargyurov]

Describe the bug
When detection is running and users press the Stop button, the error toast shows up.

To Reproduce
Steps to reproduce the behavior:

1. Go to Detection
2. Start process
3. Wait for Stop button to become active
4. Press Stop button
5. Observe error toast

Expected behavior
To stop the detection process without errors

Screenshots
N/A

System Information (please complete the following information):

• OS: Windows
• Version: 10

Additional context
The error message is null. It is probably something to do with how we handle stderr output from the backend.

[petargyurov]

We need a graceful way of handling the killing of the child process. I suspect that instead of killing it from the parent process, we could send a signal that is handled in the Python program and shuts down itself gracefully.

Related to #13