LibraryAlbum: fix removeObserver: crash in didTurnIntoFault:
- Cause: awakeFromFetch() is not always called immediately, so sometimes
  the self-observer is never setup. It then crashes when it attempts to
  remove the self-observer during the didTurnIntoFault (=~ dealloc)
  if it were never setup.
- Fix: keep track of whether the self-observer was setup through an ivar
  _isSelfObserverSetup. Then only do removeObserver: if the ivar is set.
- Fixes #38