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.
```
i think this pattern is good, i want to use it to also tag

`humanizedStack` onto errors, so in a non-verbose mode we log only those rather then spewing a million lines of esprima stack traces :package: 

cc @gavacho