[proxy] return errors when attaching new containers

I had implemented this with a promise, but was informed that as
waitForStart is only called once, we are safe to just put the error into
the channel.

test/695_proxy_failure_modes_test.sh

@@ -0,0 +1,12 @@
+#! /bin/bash
+
+. ./config.sh
+
+start_suite "Proxy failure modes"
+
+# docker run should fail when weave router is not running
+weave_on $HOST1 launch-proxy
+assert_raises "proxy docker_on $HOST1 run --rm $SMALL_IMAGE true" 1
+assert_raises "proxy docker_on $HOST1 run --rm $SMALL_IMAGE true 2>&1 1>/dev/null | grep 'Error response from daemon: weave container is not present. Have you launched it?'"
+
+end_suite