handle errors during the creation of port mapper

example:

  2013/03/22 21:42:55 Unable to setup port networking: Failed to create DOCKER chain

  (which was possibly introduced by commit 3c6b8bb88)

network.go

@@ -348,6 +348,9 @@ func newNetworkManager(bridgeIface string) (*NetworkManager, error) {
 	}
 
 	portMapper, err := newPortMapper()
+	if err != nil {
+		return nil, err
+	}
 
 	manager := &NetworkManager{
 		bridgeIface:   bridgeIface,