Diagnose packager exit failure

[anp]

Sometimes the port held by the manifest server or the React Native packager doesn't get released on exit. I suspect that this may be due to me being abusive in my dev environment, but it would be good for someone else to take a look at the exit hooks to see if there's a better way to do this.

https://github.com/react-community/create-react-native-app/blob/master/react-native-scripts/src/util/packager.js#L52:L69

[anp]

I haven't been able to reproduce this after redoing the exit hooks for the packager.