Review checker status meanings and integer values

Currently our checker statuses are mapped on a scale of 0 to 4 with Good, Warn, Alert, Info, Missing. There are a few issues with this:

• You'd be forgiven for thinking higher numbers mean higher severity, but this is not the case ("Info" should be below "Warn")
• "Missing" is unclear - we currently take this to mean "the checker ran but experienced an error, or the data it needs was somehow unavailable". Notably, a checker that did not run would have no status at all, rather than "Missing" status
• "Info" is ill-defined - in Nutshell an informational checker is one that is not actionable (iow, "here's something you might like to know, but you don't need to do anything about it"). Are we sticking with this definition or something else?

Setting priority=major because if we make any changes to these, it should be relatively soon, otherwise the amount of technical debt to pay off will constantly increase.