[RT-2859] Gracefully handle errors in Invadium

[blu3r4y]

Acceptance Criteria:

• For backend and frontend, ensure that, in the following scenarios, I am presented with proper error messages
  • Start the frontend without the backend or with an incorrect port and tell me that the backend is not reachable
  • Start the backend without exploit files - show an empty screen in the frontend that tells me "no exploit found - did you forget to mount the exploit files?"
  • Use an invalid image name in one of the exploits - show an error that the exploit could not be started because the image was not found
  • Do not mount the Docker socket and try to start an exploit - show an error that containers can not be started
  • Use a container image without a Shell and try to start the first step - show an error that EXEC failed and hint at the idea that one might has no shell in that container
  • Mount a YAML file with syntax errors into the backend - show an error in the frontend that the YAML file could not be parsed correctly for a certain ID
  • Have an exploit step return a non-zero status code - color-code the step card in red and also show me a text that this container just returned a non-zero exit code
  • Kill the container in the middle of the steps - have the frontend show me that it lost the connection to the container and I can easily restart from the overview page again
  • Start the frontend without a backend - let the frontend tell me that it can't connect to its backend
  • Start more Pods in the Ranger than CPU is available - let the frontend tell me I have to wait until a Container is stopped
• Implement test cases in the backend only to check for such patterns

Tasks:

• for each of the above, create a frontend and backend task
• for each of the above, create a test case in the backend