No clear message when config file is missing

In this case, the app loops trying to connect to a DB without any hint on what to do...
```

Exception ignored in: <function Service.__del__ at 0x7f04e6edc280>
Traceback (most recent call last):
  File "/app/API_operations/helpers/Service.py", line 168, in __del__
    if self.session is not None:
AttributeError: session
```