After upgrading to Emacs 25, I get
gnutls-negotiate: Certificate validation failed for [hostname] verification code 402 on connecting to my weechat IRC proxy, with no prompt about accepting things anyway. I already have
(add-to-list 'gnutls-trustfiles "~/.emacs.d/weechat-relay.cert")
where weechat-relay.cert is the CERTIFICATE part of the weechat IRC proxy ssl key.