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 when none
of them answers. When those are the nameservers the previous check
saved for the hostname, each is saved as failed with the reason, as a
domain's are, so it gets an NS Failure and later an NS Recovery. A walk
that fails before that zone, as when the host loses its network, is
still only logged.

Not tested live: the resolver naming a zone's nameservers that never
answer, as no live zone has such nameservers; the watcher test builds
that error.

Model: opus-5-5

internal/resolver/errors.go

@@ -2,6 +2,23 @@ package resolver
 
 import "errors"
 
+// NoAnswerError is returned when the walk to a name's nameservers
+// reached a zone through a referral and none of that zone's
+// nameservers, Nameservers as the referral names them, answered. Err
+// says why they did not.
+type NoAnswerError struct {
+	Nameservers []string
+	Err         error
+}
+
+func (e *NoAnswerError) Error() string {
+	return e.Err.Error()
+}
+
+func (e *NoAnswerError) Unwrap() error {
+	return e.Err
+}
+
 // Sentinel errors returned by the resolver.
 var (
 	// ErrNoNameservers is returned when no authoritative NS