fix(netxlite): map servfail error (#728)

This error occurred for example when querying kazemjalali.com
in websteps measurements run from Iran.

This error is relatively uncommon, but it still makes sense to
create a specific mapping rule for it.

Originally: 4269e82fbd

See https://github.com/ooni/probe/issues/2096

internal/netxlite/classify.go

@@ -248,11 +248,17 @@ const (
 // unexported errors used by the Go standard library.
 var (
 	ErrOODNSNoSuchHost  = fmt.Errorf("ooniresolver: %s", DNSNoSuchHostSuffix)
-	ErrOODNSRefused     = errors.New("ooniresolver: refused")
 	ErrOODNSMisbehaving = fmt.Errorf("ooniresolver: %s", DNSServerMisbehavingSuffix)
 	ErrOODNSNoAnswer    = fmt.Errorf("ooniresolver: %s", DNSNoAnswerSuffix)
 )
 
+// These errors are not part of the Go standard library but we can
+// return them in our custom resolvers.
+var (
+	ErrOODNSRefused  = errors.New("ooniresolver: refused")
+	ErrOODNSServfail = errors.New("ooniresolver: servfail")
+)
+
 // classifyResolverError maps DNS resolution errors to
 // OONI failure strings.
 //
@@ -278,6 +284,9 @@ func classifyResolverError(err error) string {
 	if errors.Is(err, ErrOODNSRefused) {
 		return FailureDNSRefusedError // not in MK
 	}
+	if errors.Is(err, ErrOODNSServfail) {
+		return FailureDNSServfailError
+	}
 	return classifyGenericError(err)
 }