Give more explanation if we can't find weave to talk to

plugin/ipam/driver.go

@@ -1,10 +1,12 @@
 package ipamplugin
 
 import (
+	"fmt"
 	"net"
 
 	"github.com/docker/libnetwork/ipamapi"
 	"github.com/docker/libnetwork/netlabel"
+	godocker "github.com/fsouza/go-dockerclient"
 	"github.com/weaveworks/weave/api"
 	. "github.com/weaveworks/weave/common"
 	"github.com/weaveworks/weave/common/docker"
@@ -20,7 +22,13 @@ type ipam struct {
 }
 
 func NewIpam(client *docker.Client, version string) (ipamapi.Ipam, error) {
-	resolver := func() (string, error) { return client.GetContainerIP(WeaveContainer) }
+	resolver := func() (string, error) {
+		addr, err := client.GetContainerIP(WeaveContainer)
+		if _, ok := err.(*godocker.NoSuchContainer); ok {
+			return "", fmt.Errorf("%s container is not present. Have you launched it?", WeaveContainer)
+		}
+		return addr, err
+	}
 	return &ipam{client: client, weave: api.NewClientWithResolver(resolver)}, nil
 }