Fixes for some random maps.

• Hellas_biome isn't a RM but as a JSON script it does load, and then silently fails on mapgen. Explicitly fail.
• Fix player.js when a valid position cannot exist.
• Fix unknown.js when landscape isn't set in the settings.

Reported by: vladislavbelov

Refs #6180

Differential Revision: https://code.wildfiregames.com/D4147