Scheduler fixes and logging enhancements

- Recompute the next scheduled playlist when a running playlist is
  stopped manually via the UI.  Previously this could cause the next
  playlist to be skipped until fppd was restarted.

- Prevent a fppd segfault if the web UI issues a status command just as
  fppd is starting up and the next scheduled playlist has not been
  determined yet.

- Add more detailed scheduler logging to help troubleshoot any further
  scheduler issues.