This reverts commit 3c71fa1e.  There are cases
(like failure to install a per-user extension during upgrade), where the user
may want to still see the broken extension in the Extension Manager, instead of
it "disappearing completely."  A better fix would be to roll back registration
of the broken extension completely, so that "unopkg list" would not list the
first half of its parts as "is registered: yes" and the second half as "is
registered: no."