App Building Errors
The majority of app building errors are to do with configuration issues. Double check your configuration in both your build.json and config.xml files. Also try regenerating your certificates, if you're having certificate errors. These two steps generally fix the majority of issues.
Troubleshooting an App
If you're having troubles loading your app, if you create the app in debug mode (ie. not release mode), then you can debug your app by plugging your mobile device into your computer, and debugging Android apps through Chrome Inspector, and iOS apps through Safari Developer tools.
The console logs in particular are invaluable.
Broken Evergreen Code
If you've edited your evergreen code and there are errors in it, you will need to use the Buildbot to create a new app and deploy it as usual. Basically, if the evergreen code is broken, you can't use it to push out evergreen updates.
Google Play Store Submissions
Google have some great resources for tips on Play Store submissions:
10 Tips for Developers Policy:
https://www.youtube.com/watch?list=PLWz5rJ2EKKc_ElGrEtiEXc83m1SeYu3-Q&v=62e4GGctnvY
https://play.google.com/about/developer-content-policy/