bug#75065: Upon archive download failure print the original error

[Konstantin Kharlamov <Hi-Angel@yandex.ru>]

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I was recently helping out a new Emacs user with package installation¹,
and I found an interesting thing: if you put to `package-archives` a
URL without `https` prefix, download will fail. Long story short, the
reason turns out that `package-archives` also supports local paths,
which the URL being considered as. However, Emacs never prints a
message about that, even though such message exists in the code.
Instead it just says that download failed, leaving a user wondering
why.

That happens because (package--download-and-read-archives) ignores the
exception message, and always just prints generic failure message.

This code fixes this, so now the actual failure message will be
correctly shown.

1:
https://emacs.stackexchange.com/questions/82828/is-installing-deadgrep-fron-melpa-still-possible/82829#82829

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From fb4685238726a79599f6388318916d2962da93ae Mon Sep 17 00:00:00 2001
From: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Date: Tue, 24 Dec 2024 18:16:41 +0300
Subject: [PATCH] Upon archive download failure print the original error

* lisp/emacs-lisp/package.el (package--download-and-read-archives):
upon catching exception, use the exception message as part of the
error to provide more context about the failure.
---
 lisp/emacs-lisp/package.el | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index 5f785071ea3..cb81efc71f0 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -1829,10 +1829,10 @@ Populate `package-archive-contents' with the result.
 If optional argument ASYNC is non-nil, perform the downloads
 asynchronously."
   (dolist (archive package-archives)
-    (condition-case-unless-debug nil
+    (condition-case-unless-debug err
         (package--download-one-archive archive "archive-contents" async)
-      (error (message "Failed to download `%s' archive."
-               (car archive))))))
+      (error (message "Failed to download `%s' archive. Error: %S"
+               (car archive) (cdr err))))))
 
 (defvar package-refresh-contents-hook (list #'package--download-and-read-archives)
   "List of functions to call to refresh the package archive.
-- 
2.47.1