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.
```
:+1:

This particular rejection is when the cli command is exiting (aka unhandled errors within the command/task), what you are referring to are errors that are handled within the `Watcher` (or middleware).  

IOW, I love the idea, but that is a subject for another PR I think.