Handle not found cases

[coni2k]

I was looking into whether we can improve handling "deleted/non-existing repos" cases:

• It shows a "Repo not found" message, instead of the exception
• In "generate" script, if the repo is not found, it doesn't continue with the retries

Current output for GitHub in "not found" case:

Traceback (most recent call last):
  File "..\Python39\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "..\Python39\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "..\criticality_score\criticality_score\run.py", line 584, in <module>
    main()
  File "..\criticality_score\criticality_score\run.py", line 564, in main
    repo = get_repository(args.repo)
  File "..\criticality_score\criticality_score\run.py", line 526, in get_repository
    repo = get_github_auth_token().get_repo(repo_url)
  File "..\Python39\lib\site-packages\github\MainClass.py", line 345, in get_repo
    headers, data = self.__requester.requestJsonAndCheck(
  File "..\Python39\lib\site-packages\github\Requester.py", line 315, in requestJsonAndCheck
    return self.__check(
  File "..\Python39\lib\site-packages\github\Requester.py", line 340, in __check
    raise self.__createException(status, responseHeaders, output)
github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest/reference/repos#get-a-repository"}

New output:

Repo not found: https://github.com/x/x

[inferno-chromium]

Thanks a lot!