No proper error handling if Azure filtered the response

[sentry-tum-ase]

Sentry Issue: PYRIS-K

AttributeError: 'NoneType' object has no attribute 'contents'
(19 additional frame(s) were not displayed)
...
  File "app/retrieval/lecture_retrieval.py", line 462, in run_parallel_rewrite_tasks
    rewritten_query = rewritten_query_future.result()
  File "app/retrieval/lecture_retrieval.py", line 241, in rewrite_student_query
    raise e
  File "app/retrieval/lecture_retrieval.py", line 237, in rewrite_student_query
    response = (prompt | self.pipeline).invoke({})
  File "app/llm/langchain/iris_langchain_chat_model.py", line 44, in _generate
    base_message = convert_iris_message_to_langchain_message(iris_message)
  File "app/common/message_converters.py", line 13, in convert_iris_message_to_langchain_message
    if len(iris_message.contents) == 0:

More context: Ths request was filtered by Azure due to sexual explicit content (false positive) and Pyris was not able to handle the error properly, which is why this issue occurred.
Not super critical to fix.