Plugin should validate the provided process group IDs and write a warning if there is no match

[johscheuer]

What would you like to be added/changed?

Currently our plugin just ignores provided process groups that are not matching. Instead we should provide a warning on stderr to the user to let them know that at least one of the provided process groups are not part of the cluster. In the actual warning message we should provide the information which process groups are not found in the cluster.