Add a helpful error message to openstack_upgrade_availble()
Add a helpful error message to openstack_upgrade_availble()

After spending time debugging a missing package codename, I felt it
would be useful to add an error message as to why it might be
happening.  Hopefully useful in the next round of upgrades.

Change-Id: Ie994f49847b50c1b10e4b76845099cdd09441ecd

coreycb/charms.openstack