nicer error message on 'weave status' when there is no weave container.

weaver/weave

@@ -87,7 +87,10 @@ case "$COMMAND" in
         IFACE=eth0
         ;;
     status)
-        DOCKERIP=$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' weave)
+        if ! DOCKERIP=$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' weave 2>/dev/null) ; then
+            echo "There is no weave container." >&2
+            exit 1
+        fi
         if [ -z "$DOCKERIP" ] ; then
             echo "The weave container is not running." >&2
             exit 1