overhaul action levels to log levels

[kmasiello]

Action levels are more appropriately described as flags or threshold levels. Align flags with log levels for further downstream actions.

warn_at-> warn
stop_at -> error
notify_at -> critical

Aligns with decisions made in python pointblank. posit-dev/pointblank#69

[rich-iannone]

Thanks for the issue! Yeah, this should absolutely be done. It's a breaking change like in py-pointblank but the R version has had W/S/N for years so this'll have to be more like a staged migration.