Error message if missing references in Config Files

At the moment, if a Config file e.g. Moon.cfg) references a non-existent sub-config (e.g. Fra Mauro.cfg in a SURFBASE clause), then orbiter will crash horribly with no log, and no message. For this particular case, the crash occurs when the camera dialog runs, or the "spaceport" tab is clicked in the "Object Info" dialog.

It would be much better if there was a log message pointing out which Config file had the bad reference in the log file, and terminating cleanly.