error out if we're not able to move the cgroup pid into the default group

Bug https://bugzilla.redhat.com/show_bug.cgi?id=1082980

During the oo-admin-gear cleaning we may encounter a situation where a pid that does not exist or is a zombie process is still within the cgroup process list and we won't be able to move it out.

In this case, we need to error out the problematic process id and let the sysadmin deal with it.