Better stack trace and removed bug created in my last pull request.

In the last pull request (1.2.1), I removed the code that threw an exception upon test errors. But I misinterpreted how the function works so this means that version 1.2.1 made the runner to continue the tests instead of ending it on failure. I fixed it in this version with a system.exit(1) instead. I think this is a better behavior than outputting a stack trace that only points to the sbt plugin.

I also made some changes to the stack trace as I found some exceptions that was not taken care of properly after my last changes.