xlets: Add better error handling and reporting.

    - At the end of Cinnamon startup, show a notification if any applet,
      desklet or extension has failed to load.
    - In spices, show an 'error' status for items that *should* be running,
      but are not actually currently running in Cinnamon.  Prevent adding more
      instances or configuring these items.  Hopefully this should prevent
      issues such as #3568.
    - In spices, detect when a newly-enabled xlet has failed to load, disable it,
      and notify the user, with an option to view log files (this will need fleshed
      out for other distros.)