A long time ago Magnus Henoch <mange@freemail.hu> wrote:

> open-tls-stream in tls.el returns nil on failure (gnutls-cli binary
> not found, connection refused, etc).  open-network-stream signals an
> error when it can't connect.  Is there any reason open-tls-stream
> shouldn't do the same, instead of just returning nil?

As nobody answered Magnus I made a patch, it seems to work like
open-network-stream on errors, but it's hardly tested.