Add error percentage to cpu and ram monitor

[sangteak601]

At the moment, it is not possible to configure cpu_monitor or ram_monitor to publish ERROR status. It would be good to have ERROR state. It would allow us to take actions before program exhibits unexpected behaviour.

Also, cpu_monitor was updated to compare threshold to average cpu usage rather than individual cpu usage. This is aligned with how ram_monitor works and makes more sense, as in most cases users are more interested in average usage than individual usage.