nicer error on 'weave stop' and make it idempotent

weaver/weave

@@ -135,8 +135,9 @@ case "$COMMAND" in
         exit 0
         ;;
     stop)
-        docker kill weave >/dev/null
-        docker rm weave >/dev/null
+        if ! docker rm -f weave >/dev/null 2>&1 ; then
+            echo "Weave is not running."
+        fi
         if command_exists conntrack
         then
             conntrack -D -p udp --dport 6783 >/dev/null 2>&1