IntelliJ test-runner gets confused if the Java compiler errors

[danwallach]

ClassNotFoundError, etc, rather than stopping.

• tweak build.gradle to convert ErrorProne errors into warnings
• tweak build.gradle to make builds fail correctly
  • but somehow the autograder still gets to run?

[danwallach]

Probably needs a rev to RiceChecks: if it doesn't see unit test output, but sees compiler warnings, it can infer that the compile failed, and it should give a zero grade.