Explicitly call out 412 statuses

Author: kpshek

docs/specification/1.0.md

@@ -427,6 +427,7 @@ For successful responses, CDS Services SHALL respond with a 200 HTTP response wi
 Code | Description
 ---- | -----------
 `200 OK` | A successful response.
+`412 Precondition Failed` | The CDS Service is unable to retrieve the necessary FHIR data to execute its decision support, either through a prefetch request or directly calling the FHIR server.
 
 CDS Services MAY return other HTTP statuses, specifically 4xx and 5xx HTTP error codes.