Establish behavior for installed content no longer present in the catalog

[dtfranz]

Continuation of our discussion here.

We need to both capture existing behavior as well as decide on new behaviors for how we inform users that their installed content is no longer present in the catalog.

This is particularly relevant to any deprecated content, because users with deprecated content that miss a catalog update marking the content as such before a prune will not receive the "Deprecated" status updates.

What we've proposed so far is informing users in this situation that since their content is no longer in the catalog it may be deprecated. For instance:

entry, ok, err := entryForThisSubscription(sourceProvider, sub)
if err != nil {
   setDeprecationConditionsUnknown(sub, err)
} else if !ok {
   setDeprecationConditionsUnknown(sub, errors.New("could not find catalog entry matching subscription"))
} else {
   setDeprecationConditionsFromEntry(sub, entry)
}