Exit status code should not be 0 on error

[OndraM]

When error occurs in any test, the Steward exit code should not be 0.

For example on Travis CI in our https://github.com/lmc-eu/steward-example - if you don't read logs, you won't find that some test is failing.

On the other side - as the test tends to be unstable on some circumstances, and this "non-zero" exit code of the process would fail whole build eg. on Jenkins, this may not be completely needed behavior, because even with failing test you want the build to "succeed", so Jenkins would log its history, process JUnit graphs etc.