fixing error handling on mean.serve()

fixing error handling so that we're catching http or https listening failures due to unable to bind to address or other reasons, then exiting and making sure that the rest of the mean app doesnt keep running as mean.serve() is unable to complete the job
I don't think the `undefined` is necessary. The last param will be assumed as callback.
```
httpServer.listen(config.http ? config.http.port : config.port, config.hostname,readyApp);
```
^ should be equivalent.