don't duplicate output on error in with_container_netns

This does not cause any problems right now, since all the call sites
swallow stdout on error. But it is wrong nevertheless.

weave

@@ -191,9 +191,9 @@ with_container_netns () {
             echo "Failure during network configuration for container $CONTAINER:" >&2
             cat $IP_TMPFILE >&2
         fi
+    else
+        cat $IP_TMPFILE
     fi
-
-    cat $IP_TMPFILE  # no error - send the output to stdout
     rm -f $IP_TMPFILE /var/run/netns/$NETNS
     return $STATUS
 }