socket_util.py: Make set_dscp() python 2.4.3 compatible.

There is no 'errno' field in socket.error. Instead use the
get_exception_errno() function to get the error number.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>