clarify "instrepo not found" error (#980818 et. al)

The existing error message didn't make clear that you probably just need
to try again later ('cuz the mirrors are overloaded and/or you're
jumping the gun on the release).
Update TODO
README: add details about network upgrades

This should be helpful for repo maintainers and/or anyone else who is
curious/confused about how fedup finds the repos and images it uses.