fail gracefully when listen socket can't do IPv6

fixes bug introduced in 4857c99c7bbf85f28c490f7f74f387c7d6e01a08
(listen on ipv6 as well as ipv4)

Signed-off-by: Thomas Stilwell <stilwellt@openlabs.co>
Merge pull request #335 from benemorius/ipv6-fix

fail gracefully when listen socket can't do IPv6