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/internal/generrno/main.go

@@ -158,6 +158,7 @@ var Specs = []*ErrorSpec{
 	NewLibraryError("DNS_refused_error"),
 	NewLibraryError("DNS_server_misbehaving"),
 	NewLibraryError("DNS_no_answer"),
+	NewLibraryError("DNS_servfail_error"),
 	NewLibraryError("EOF_error"),
 	NewLibraryError("generic_timeout_error"),
 	NewLibraryError("QUIC_incompatible_version"),