Tolerate missing elpa-packages.eld files

Tolerate missing elpa-packages.eld files

* lisp/emacs-lisp/package-vc.el
(package-vc--download-and-read-archives): Replace
'condition-case-unless-debug' with a regular 'condition-case'.

It appears a few people using third-party archives that don't serve
package specifications have been having issues with package-vc, when
toggle-on-error is enabled.  In their case, package-vc would raise an
error in its first invocation, but it would go on working normally
afterwards.  As this behaviour is confusing and the user can't do much
about a missing elpa-packages.eld to begin with, we satisfy ourselves
with printing out a message and continuing on.