fix(*): Handling of unavailable services

Send http code 503 for not running services.

This improves messaging when a service is temporarily unavailable and fails app health checks pinging `/` when all instances of an app are offline.

Example with a stopped controller you receive now:
```
$ deis apps
503 Service Temporarily Unavailable
<html>
<head><title>503 Service Temporarily Unavailable</title></head>
<body bgcolor="white">
<center><h1>503 Service Temporarily Unavailable</h1></center>
<hr><center>nginx/1.6.2</center>
</body>
</html>
```
Fixes #2161