Send explicit RuntimeError on response operations after session closing

[asvetlov]

Extracted from aio-libs chat

aiohttp.client_exceptions.ClientConnectionError: Connection closed when a response is significantly large: https://pastebin.com/NmmnqhVt though I am new to async stuff so likely doing something wrong...

this is because you move response out of session context and session closes connection

Yes, I see.
First snippet uses aiohttp incorrectly, I don't think we need fix our code.
Reading response after session closing is mistake.
Well, we could raise another explicit exception like RuntimeError but I doubt if we need it.

We have to raise exception, otherwise it is very hard to find source of the problem especially for developers new to aiohttp

See also #2193

[joaosimbiose]

Thanks so much for sharing this, helped me solving my problem