Fix IgnoreSignals on socket.error

Fix IgnoreSignals on socket.error

Some confusion arose handling EINTR on this function: in python 2.6
socket.error is an IOError, and thus:
  - It's an EnvironmentError
  - It has an .errno member

In 2.4 and 2.5 it's not, and so its errno variable must be extracted
from the args tuple. This patch fixes both the function, and the
unittests.

This is a cherry-pick of master commit
965d0e5ba37f3e88aa38230177ad1c66814bf927 with the portions not relevant
to 2.1 removed (changes to the RetryOnSignals function).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>