fix(netxlite/dns): more stricly mirror stdlib error strings (#513)

This diff attempts to modify the errors reported by our custom
resolver by matching more strings from the stdlib.

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/tls.go

@@ -121,7 +121,7 @@ type TLSHandshaker interface {
 	//
 	// QUIRK: The returned connection will always implement the TLSConn interface
 	// exposed by this package. A future version of this interface will instead
-	// return directly a TLSConn and remove the ConnectionState param.
+	// return directly a TLSConn to avoid unconditional castings.
 	Handshake(ctx context.Context, conn net.Conn, config *tls.Config) (
 		net.Conn, tls.ConnectionState, error)
 }