codegen: improve unknown enum value error message

new error message:
	FATAL Exception: unknown enum value 'OLD_IDENTIFIER' encountered. valid are: TERRAIN,SHADOW,RUBBLE,UNIT_LOW,FISH,CRATER,UNIT,BLACKSMITH,BIRD,PROJECTILE
	---
	If this is an inconsistency due to updates in the media converter, `make media` should fix it
	---
Merge pull request #145 from franciscod/unknown-enum-value

codegen: improve unknown enum value error message