BPPH.supersede shouldn't ever supersede obsolete or deleted publications

[William Grant]

IPublishing.supersede is meant to assert that the publication is Pending or Published. However, BinaryPackagePublishingHistory.supersede makes an exception for architecture-independent binaries.

While the idea behind this exception is sane (arch-indep binaries supersede each other atomically, so calling supersede() on a Superseded publication needs to be OK), it should ensure that the publication is Superseded. As it stands, it will mark a Deleted or Obsolete publication as Superseded -- but such a situation can only arise due to a bug, so it should crash and burn.