Hi all,
I've got a problem: when using (gnutls-negotiate :verify-error t) to initialize a SSL/TLS
connection , server has an expired certificate cause gnutls-boot signal
an error "Invalid format operation %u", apparently it's not related to
gnutls-error. So digging into the source
code, here is the error signal:

src/gnutls.c:1511

      if (verify_error_all
          || !NILP (Fmember (QCgnutls_bootprop_trustfiles, verify_error)))
        {
	  emacs_gnutls_deinit (proc);
	  error ("Certificate validation failed %s, verification code %u",
		 c_hostname, peer_verification);
        }

After checking elisp manual and source code, function Fformat(error call
it to format string) does not
support '%u', here should using '%d' instead.

I made a simple patch for this: