Improve usability of the ci-checkstyle-javadoc.sh script for local use

[slarse]

The script currently just runs in CI mode, and reports the number of errors. We need to make it possible to also show errors, and to run it locally on the current working tree. In its current state, it's very difficult to a) figure out what to fix when it fails a build, and b) using it locally to actually improve the docs rather than just not make them worse!

I'll fix this ASAP.