Feature/921 ignore async handle request socket error

after testing and discussion on our side [see release/19.1.1+laterpay2](https://github.com/laterpay/gunicorn/pull/2), this seems to be the right fix for benoitc/gunicorn#921.

I first tried to implement @tilgovi's suggestion in laterpay/gunicorn@301c3fdfadf402935c8ca599b17bf1802ebe6a8e but that lead to the stacktrace detailed in [this comment](https://github.com/laterpay/gunicorn/pull/2#issuecomment-61808305). The patch in this PR leads to no more logged errors on our servers, but somebody more familiar with gunicorn internals should definitely see whether there are unhandled edge cases.

(CC @doismellburning fyi)