Crash on missing scenario components

[CelticMinstrel]

The scenario editor (and presumably the game as well) currently fails to load a scenario that is missing some components inside the package. In some cases it even crashes.

For example, deleting a town's talk file will cause a crash. Deleting the special nodes file won't crash but will fail to load.

The scenario editor should ideally be able to load what exists and substitute the missing pieces with nothing. If the special nodes file is missing, there are just no nodes defined. If the talk file is missing, the personalities are all blank and there are no nodes defined.

Of course, loading a partially valid scenario in the game should not happen, but it should at least not crash.