Fix "weave ps" output when weave is not running

The #2418 has changed a behaviour of "weave ps" which outputs
"Link not found" in the case of weave not running. The previous
behaviour was to output an empty line.

net/netns.go

@@ -1,12 +1,15 @@
 package net
 
 import (
+	"errors"
 	"runtime"
 
 	"github.com/vishvananda/netlink"
 	"github.com/vishvananda/netns"
 )
 
+var ErrLinkNotFound = errors.New("Link not found")
+
 // NB: The following function is unsafe, because:
 //     - It changes a network namespace (netns) of an OS thread which runs
 //       the function. During execution, the Go runtime might clone a new OS thread
@@ -42,6 +45,9 @@ func WithNetNSLinkUnsafe(ns netns.NsHandle, ifName string, work func(link netlin
 	return WithNetNSUnsafe(ns, func() error {
 		link, err := netlink.LinkByName(ifName)
 		if err != nil {
+			if err.Error() == errors.New("Link not found").Error() {
+				return ErrLinkNotFound
+			}
 			return err
 		}
 		return work(link)

prog/weaveutil/addrs.go

@@ -5,7 +5,9 @@ import (
 
 	"github.com/fsouza/go-dockerclient"
 	"github.com/vishvananda/netlink"
+
 	"github.com/weaveworks/weave/common"
+	weavenet "github.com/weaveworks/weave/net"
 )
 
 func containerAddrs(args []string) error {
@@ -21,6 +23,9 @@ func containerAddrs(args []string) error {
 
 	pred, err := common.ConnectedToBridgePredicate(bridgeName)
 	if err != nil {
+		if err == weavenet.ErrLinkNotFound {
+			return nil
+		}
 		return err
 	}