Raise exception when server response is None.

[justinschembri]

Summary

Currently, the postRequest function only handles 403 errors explicitly. For other error responses (e.g., 500 — which I tend to encounter frequently!), the function attempts to subscript a NoneType object, resulting in an ungraceful exception. This PR introduces an early exception for such cases to improve robustness and error visibility.

Typical Error

2025-04-05 20:03:04: ERROR - code=500, reason=Internal Server Error, body=b'{"error":{"code":500,"message":"Internal Server Error"}}'
Traceback (most recent call last):
  <out of scope code omitted>
  File "/app/src/sensorthings_utils/netatmo.py", line 97, in _extract
    weather_station_data = ln.WeatherStationData(AUTHENTICATION)
  File "/app/.venv/lib/python3.13/site-packages/lnetatmo.py", line 460, in __init__
    self.rawData = resp['body']['devices']
                   ~~~~^^^^^^^^
TypeError: 'NoneType' object is not subscriptable