Exit properly from EADDRINUSE.

Closes #1314.

```
_____ember-cli-test master % ember server
version: 0.0.39-exit-properly-c791a21573

Livereload failed on port 35729.  It is either in use or you do not have permission.
```
FYI - This is done here: https://github.com/stefanpenner/ember-cli/blob/master/lib/cli/cli.js#L60.