NSM has better UX when an invalid/expired certificate is encountered: it
prompts the user to decide what to do with it. If gnutls-verify-error is
non-nil, gnutls either kills or hangs the connection. This is (mostly)
acceptable in noninteractive sessions, where I can more tightly control
outgoing connections, but not in interactive sessions where I stand a
higher chance of stepping on the user's toes instead.
Ref: emacs-circe/circe#405