* Redesign HealthStatus (again)
This change brings back the ability to return Healthy/Degraded/Unhealthy
in a HealthCheckResult. We tried making this pass/fail in 2.2.0-preview3
and folks writing health checks for their own use pointed out (rightly
so) that it was too limited.
It's still possible for the app developer to configure the failure
status of a health check, but it requires the health check author to
cooperate.
I also got rid of HealthStatus.Failed since it raises more questions
than it answers. It's really not clear that it's valuable for a health
check for behave different when throwing an unhandled exception.
We would still recommend that a health check library handle exceptions
that they know about and return `context.Registration.FailureStatus`.
Ryan Nowak