Added a little debugging to make it easier to locate EADDRINUSE errors.

For example.

```
  slacker 63310 starting listener on port 9096 +0ms

events.js:71
        throw arguments[1]; // Unhandled 'error' event
                       ^
Error: bind EADDRINUSE
    at errnoException (net.js:770:11)
    at net.js:944:26
    at Object.cluster._getServer [as 1:1] (cluster.js:555:5)
    at handleResponse (cluster.js:149:41)
    at respond (cluster.js:170:5)
    at handleMessage (cluster.js:180:5)
    at process.EventEmitter.emit (events.js:126:20)
    at handleMessage (child_process.js:270:12)
    at child_process.js:334:7
    at process.handleConversion.net.Native.got (child_process.js:90:7)
```