feed: Add herror to _SOCKET_ERRORS and remove reason handing

We don't log the reason, so trying to extract it just gives room for
errors to creep in.  Luckily, we'll be able to drop the whole
_SOCKET_ERRORS thing when we move to Python >= 3.3, because following
PEP 3151 the socket errors became subclasses of OSError.

Reported-by: Matt Bordignon <matthew@bordignons.net>
Signed-off-by: W. Trevor King <wking@tremily.us>