bug#31339: 25.3.50; package-list-packages does not provide useful error when gnutls-cli is missing

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Lars Ingebrigtsen <larsi@gnus.org> writes:

> Glenn Morris <rgm@gnu.org> writes:
>
>> Does an end-user who does not compile Emacs themselves receive any
>> notification about avoiding tls.el?
>
> Hm...  It's in lisp/obsolete.

Note: it wasn't when Glenn posted that.  See Bug#31457 "27.0.50; Move
starttls.el and tls.el to lisp/obsolete/".

> Does Emacs have a general method to warn
> about requiring files in that directory?

Emacs does warn about requiring files from obsolete.  Users loading
tls.el will now get this in *Messages*:

    Package tls is obsolete!

It's not super visible though, opening a connection with tls.el produces
several other messages too, so it looks something like this:

    Opening TLS connection to ‘example.com’...
    Opening TLS connection with ‘gnutls-cli --x509cafile 
/etc/ssl/certs/ca-certificates.crt -p 443 example.com’...
    Package tls is obsolete!
    Opening TLS connection with ‘gnutls-cli --x509cafile 
/etc/ssl/certs/ca-certificates.crt -p 443 example.com’...done
    Opening TLS connection to ‘example.com’...done