task/cgroup - clarify messages when job/step memory[+swap] limit is hit.

There are out of memory conditions where spikes of memory usage hit the
limit set. When this happens (failcnt > 0), the Kernel might be able
to reclaim unused pages and the process can continue without oom-killer
actually killing the process. This may or may not result in an app
problem, thus we want to better clarify the message.

A separate bug will track the potential addition of a new feature to
better discern memory limits being hit from oom-killer actually killing
the process. There are mechanisms to register a notifier through the
cgroup.event_control control file, so that the application can be
notified through eventfd when OOM-Killer actually kills the process.

Bug 3820.