Make test code print an informational error message if it doesn't find 'DEIS_CONTROLLER_URL' in the environment

After https://github.com/deis/workflow-e2e/pull/157, the tests look for `DEIS_CONTROLLER_URL` in the environment to determine what server to talk to in order to run tests. This variable can be set to a variety of different values, and the test should explain the options:
- If you're inside a Kubernetes pod inside a namespace with a full Deis install, set the value to the internal router address: `DEIS_CONTROLLER_URL=${DEIS_ROUTER_SERVICE_HOST}:${DEIS_ROUTER_SERVICE_PORT}`
- If you're not inside a Kubernets pod, set the value to the external router address. This value depends on what kind of ingress server you have set up. It might be a cloud load balancer or otherwise. Assuming you have a public IP for the router: `DEIS_CONTROLLER_URL=deis.${EXTERNAL_ROUTER_IP}.xip.io` (or you can use `.nip.io`, or your own domain if you've configured one...)