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/dnsencoder_test.go

@@ -8,7 +8,6 @@ import (
 )
 
 func TestDNSEncoder(t *testing.T) {
-
 	t.Run("encode A", func(t *testing.T) {
 		e := &DNSEncoderMiekg{}
 		data, err := e.Encode("x.org", dns.TypeA, false)