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

[Stefan Monnier]

>>>    (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))))))
>>>  
>> Stefan and Philip, is this okay to install?

I agree with the idea behind the patch, but printing just `(cdr err)`
doesn't seem right, it should print the whole of `err`.

> It seems harmless, I am just uncertain if we should prefer %S or %s to
> format the error message.

`%s` to print `err` or `(cdr err)` would be wrong, since `%s` is for use
with strings rather than lists.  IOW, IMO, it should be either

    ...%S" ... err)

or

    ...%s" ... (error-message-string err))

where the first is a bit more "debugging/developer" friendly and the second
is a bit more "user" friendly.


        Stefan