Set explicit order, default for CheckStatus enum members

[irontoby]

I would like to propose setting an explicit default value for the CheckStatus enum, so that uninitialized variables get the expected value of Unknown. This will communicate to developers that they can count on this default value.

I would also like to propose ordering the enum values from lowest to highest urgency, similar to e.g. Microsoft.Extensions.Logging.LogLevel. This makes it trivial to do things like find the most critical result from this collection (var worst = checks.Max()), find all checks which are warning or worse (var alerts = checks.Where(ch => ch >= CheckStatus.Warning)), sort them by urgency, etc. This is needed when a single collection point gathers checks from multiple locations and "bubbles them up" for display, alerting, etc.

I could not find anywhere in the solution that is currently depending on the order or explicit values from this enum, so there should be no negative impact from this change.