3.7 allows cron concurrency levels to be set to anything including zero, handle this nicely

[brendanheywood]

A legitimate case for concurrency to be zero would be prior to an outage:

catalyst/moodle-auth_outage#149

I don't know what the current behavior is exactly, but in this case we want to make sure that we still warn but don't go critical if concurrency is zero. Instead have a special warn label saying 'Cron has been throttled, outage likely imminent'