Add response context to errors raised by iter_content

[millerdev]

Errors raised by Response.iter_content did not have request/response context.

I couldn't figure out how to simulate StreamConsumedError or ConnectionError as raised from that context, and I could not find any other tests that simulate those errors either. However, the additions are so trivial and other tests were updated to test the very same thing, so I'm confident in the changes.

Except for StreamConsumedError ~this introduces a circular reference between the Response and error objects because of <Response>._error. One way I thought of solving that issue is to break the cycle with a weakref, but that seems annoying since it would likely involve changing the implementation of RequestException in a way that may make it lose references.~ Another option would be to pass request=self.request instead of response=self. I added https://github.com/psf/requests/pull/5323/commits/8bb881fe425da8e2b5043da73462e6128d728506 to do that instead since that seems like a reasonable idea.