Make mesos pre/post scripts tolerate it when mesos isn't running

I have this situation all the time where I screw up mesos in such a way that it crashes, bad config, whatever.

So when I try to fix my issue by upgrading mesos (with puppet, via the package), I cannot, because set -e.

This makes the scripts slightly more tolerant of these situations by only trying to stop the service if the process is actually in the process tree.