Fix: Wrong error message

[stefantds]

I might be missing something, but in my understanding the checks and the failure message here are incorrect. The PR proposes a possible way to fix this.

Context: while working on https://github.com/stefantds/go-epi-judge I got to look at some Java test code a bit closer. I've found some small potential improvements. I am opening a separate PR for each. Please have a look if there is something that can be improved.