highlight the need for weave expose in CNI/k8s (#2411)

* highlight the need for `weave expose` in CNI/k8s

Fixes #2405

* Extend error message

* rearranged the sentences

weave launch <peer hosts> seems a bit orphaned here maybe include the weave connect command as well, since it's being mentioned.

plugin/net/cni.go

@@ -139,7 +139,7 @@ func findBridgeIP(bridgeName string, subnet net.IPNet) (net.IP, error) {
 		return nil, fmt.Errorf("Could not find %q bridge", bridgeName)
 	}
 	if len(netdevs[0].CIDRs) == 0 {
-		return nil, fmt.Errorf("Bridge %q has no IP addresses", bridgeName)
+		return nil, fmt.Errorf("Bridge %q has no IP addresses; did you forget to run 'weave expose'?", bridgeName)
 	}
 	for _, cidr := range netdevs[0].CIDRs {
 		if subnet.Contains(cidr.IP) {