watcher: a hostname whose zone's nameservers all stop answering gets NS Failure (closes #284)

When none of the nameservers of a hostname's zone answered, the walk to
them failed and the check was only logged. The walk now returns a
NoAnswerError naming the nameservers of the zone it reached through a
referral when none of them answers. When those are the nameservers the
previous check saved for the name, each is saved as failed with the
reason, so it gets an NS Failure and later an NS Recovery. A domain with
no delegation of its own, whose NS lookup is that same walk, gets the
same. A walk that fails before that zone, as when the host loses its
network, is still only logged.

Model: opus-5-5

internal/watcher/interfaces.go

@@ -20,7 +20,10 @@ type DNSResolver interface {
 	) ([]string, error)
 
 	// LookupAllRecords queries all record types for a hostname,
-	// returning each nameserver's response keyed by nameserver.
+	// returning each nameserver's response keyed by nameserver. When
+	// the walk to the nameservers reached a zone through a referral and
+	// none of that zone's nameservers answered, the error is a
+	// *resolver.NoAnswerError naming them.
 	LookupAllRecords(
 		ctx context.Context,
 		hostname string,