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

[Eric Hanchrow]

I typed "M-x package-list-packages", and saw complaints about ...

Opening TLS connection to ‘melpa.org’...
Opening TLS connection with ‘gnutls-cli --x509cafile
/etc/pki/tls/certs/ca-bundle.crt -p 443 melpa.org’...failed
Opening TLS connection with ‘gnutls-cli --x509cafile
/etc/pki/tls/certs/ca-bundle.crt -p 443 melpa.org --protocols ssl3’...failed
Opening TLS connection with ‘openssl s_client -connect melpa.org:443
-CAfile /etc/pki/tls/certs/ca-bundle.crt -no_ssl2 -ign_eof’...failed
Opening TLS connection to ‘melpa.org’...failed
Package refresh done
Failed to download ‘melpa’ archive.

I eventually realized that the problem was simply due to my not having a
gnutls-cli executable!  But it took me longer to realize that than it
should have.  OK, I confess; it took me _two years_ to realize that
(seriously).  I'd just assumed that the problem was some subtle certificate
mismatch or something that I'd never understand, and I put up with it.

Would it be possible to emit a message describing _why_ gnutls-cli failed?


In GNU Emacs 25.3.50.1 (x86_64-unknown-linux-gnu)
   of 2018-01-26 built on dev-dsk-erichan-2b-i-833ca944.us-west-2.amazon.com
Repository revision: 79bbd51b1893b65626aeffb13ce1784906fc642d
Configured using:
   'configure --without-x'

Configured features:
JPEG SOUND GPM NOTIFY LIBSELINUX LIBXML2 ZLIB

Important settings:
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    value of $LANG: en_US.UTF-8
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Major mode: Messages

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Recent messages:
reference to free variable ‘Opening’
Mark set [2 times]
Auto-saving...done
Mark set [3 times]
C-M-y is undefined
Mark set [2 times]
Saving file /local/home/erichan/homedir/.emacs.d/init.el...
Wrote /local/home/erichan/homedir/.emacs.d/init.el
No obsolete buffers; did nothing.
No projects needed to be removed.

Load-path shadows:
/home/erichan/.emacs.d/elpa/let-alist-1.0.5/let-alist hides
/home/erichan/git-repositories/3rd-party/emacs/lisp/emacs-lisp/let-alist

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