makecache.cron: Explicitly ignore errors from cron makecache

I was going to change this to use "exec", but realized that the
current behavior may be somewhat intentional.  Document it.
makecache.cron: Only run if we have a cache at all

In the case of an upgraded system from Fedora 20 where dnf wasn't
installed, and the user continues to use yum (or PackageKit), we'll be
pointlessly downloading metadata even if dnf is not in use.

See: https://lists.fedoraproject.org/pipermail/devel/2014-December/205506.html

Reported-by: Hedayat Vatankhah <hedayat.fwd@gmail.com>