resolver: a nameserver with a failed record type is not nodata (closes #253)

classifyResponse set nodata when every record type that answered had
no records, even when another type's query got no usable reply. That
type is listed in FailedTypes and its records are unknown, so the
nameserver has not said it has none. It now stays ok, the status
README describes for a nameserver with a failed type, and nodata is
set only when no type failed. The watcher saved nodata as ok already,
so saved state is unchanged; the live test that rejects nodata no
longer fails when one of a nameserver's queries is lost.

Model: opus-5-5

internal/resolver/iterative.go

@@ -941,7 +941,9 @@ func isTimeout(err error) bool {
 
 // classifyResponse sets the nameserver's status. One that answered no
 // record type has failed, and Error says why; one that answered some has
-// the status of those answers.
+// the status of those answers. It has no data only when every type
+// answered with no records: a type in FailedTypes may have records, so a
+// nameserver with one stays ok.
 func classifyResponse(resp *NameserverResponse, state queryState) {
 	switch {
 	case state.gotNXDomain && !state.hasRecords:
@@ -961,7 +963,8 @@ func classifyResponse(resp *NameserverResponse, state queryState) {
 	case state.gotReferral && !state.answered:
 		resp.Status = StatusError
 		resp.Error = "server returned a referral"
-	case !state.hasRecords && !state.gotNXDomain:
+	// An NXDOMAIN reply with no records was taken by the first case.
+	case !state.hasRecords && len(resp.FailedTypes) == 0:
 		resp.Status = StatusNoData
 	}
 }