The current koji download code throws exceptions if any given build doesn't have rpms of the currently desired arch. This causes problems if only a single arch is used (usually 'x86_64' or 'i686' - 'noarch' builds like most perl or python packages will fail because there are only 'noarch' builds). The 'missing' rpms will throw an exception and kill the entire job in most cases where koji is downloading rpms and 'noarch' is not the arch used.

This could be solved in multiple ways:
* always add 'noarch' to koji downloads
* I think this is probably the better way to do things even though it may cause more downloads than are strictly required. I can't think of a situation where having noarch builds would cause problems.
* catch exceptions about 'missing' rpms and log a WARNING instead of killing the entire job
* I'm not so sure this is a great idea - there are legitimate cases where we do want errors for missing rpms and I suspect that keeping the exception optional will add more complexity than its worth.

Whichever solution is used, the koji download code needs to be less fragile when a single arch is being used.