Add ErrorProne Code Quality Tool

[vorburger]

The goal of this issue is to add https://errorprone.info to this project.

vorburger/ch.vorburger.exec#119 can serve as inspiration for how to do this.

I'm personally not going to work on this (no interest), but welcome and will merge a PR for it.

@mosesn perhaps this interests you, or one of your colleagues, or anyone else reading along here.

[vorburger]

#775 makes a start with this... an extension would be to add -XepAllDisabledChecksAsWarnings.