Ensure a server failure results in non-zero launcher exit status

add a try catch on myserver.start(), add exit status 2 to close_connection, force failure in lorenz to ensure CI fails.

As this MR is only effective if it fails the pipeline, I am linking here to the failed pipeline example:

https://gitlab.inria.fr/melissa/melissa-combined/-/pipelines/666594

And then I "fixed" the code that caused the lorenz failure, so that the pipeline passes.