Handle 404 more gracefully / explicitly

[mausch]

Hi and thanks for this project!

Describe the bug
When getting a 404 I get an unrelated stack trace as the code doesn't seem to handle the response status:

Traceback (most recent call last):
  File "/home/mauricio/prg/elevate/elevate-chatbot/.mamba/envs/chatbot/lib/python3.10/site-packages/gql/client.py", line 285, in execute_async
    return await session.execute(
  File "/home/mauricio/prg/elevate/elevate-chatbot/.mamba/envs/chatbot/lib/python3.10/site-packages/gql/client.py", line 1220, in execute
    result = await self._execute(
  File "/home/mauricio/prg/elevate/elevate-chatbot/.mamba/envs/chatbot/lib/python3.10/site-packages/gql/client.py", line 1126, in _execute
    result = await asyncio.wait_for(
  File "/home/mauricio/prg/elevate/elevate-chatbot/.mamba/envs/chatbot/lib/python3.10/asyncio/tasks.py", line 447, in wait_for
    return fut.result()
  File "/home/mauricio/prg/elevate/elevate-chatbot/.mamba/envs/chatbot/lib/python3.10/site-packages/gql/transport/aiohttp.py", line 325, in execute
    if "errors" not in result and "data" not in result:
TypeError: argument of type 'NoneType' is not iterable

To Reproduce
Steps to reproduce the behavior:
Point any gql query to a server that responds with 404.

Expected behavior
An explicit exception mentioning that the status was 404.

System info (please complete the following information):

• OS: Linux
• Python version: 3.10
• gql version: 3.4.1
• graphql-core version: 3.2.3

[leszekhanusz]

404 errors should raise a TransportServerError.

Your problem seems that the 404 answer received from the backend is empty and it is causing a bug which has already been fixed in version v3.5.0a0

Upgrading to the latest pre-release version v3.5.0b4 should fix your problem.

You should also consider adding an html answer in your backend for your 404 response.