Error is unclear when `app_id`, `installation_id` and/or private key aren't consistent

[casparvl]

I got this error when trying to start the event_handler.sh

Configuration check: PASSED
Traceback (most recent call last):
  File "/usr/lib64/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib64/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/casparvl/EESSI/bot-instance/eessi-bot-software-layer/eessi_bot_event_handler.py", line 716, in <module>
    main()
  File "/home/casparvl/EESSI/bot-instance/eessi-bot-software-layer/eessi_bot_event_handler.py", line 700, in main
    github.connect()
  File "/home/casparvl/EESSI/bot-instance/eessi-bot-software-layer/connections/github.py", line 90, in connect
    return github.Github(get_token().token)
  File "/home/casparvl/EESSI/bot-instance/eessi-bot-software-layer/connections/github.py", line 64, in get_token
    _token = github_integration.get_access_token(installation_id)
  File "/home/casparvl/EESSI/bot-instance/venv_eessibot/lib64/python3.6/site-packages/github/MainClass.py", line 859, in get_access_token
    status=response.status_code, data=response.text
TypeError: __init__() missing 1 required positional argument: 'headers'

on a newly deployed bot instance. It turned out I updated my installation_id and private_key correctly, but forgot to update app_id. Of course, that's my own silly mistake, but the error is rather cryptic. But maybe we could throw a more informative message, e.g. suggestion to check these three items in the config (and make sure they are consistent, i.e. belong to the same installed app).

[trz42]

Hmm, wouldn't it be better to validate the configuration?

Also see #289