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.
```
seems like our error printing code should print this as the error, but without a stack? So maybe a  PrettyError object, where we assume its "message" is sufficient for printing, and no stack