Require --product for Fedora upgrades to F21

From Fedora 21 onward, there are multiple "products", and every Fedora
system needs a product. So this implements the checks to require that you
use '--product' if you're upgrading to Fedora 21.

I don't love version+distro specific code, but apparently there's just
No Other Way To Solve This Problem, so there it is.

The error message text comes from this mailing list thread:
https://lists.fedoraproject.org/pipermail/devel/2014-October/203003.html