convert: improve unknown enum value error message

In order to prevent further issues due to changes like #119 (see #144, #69 [a comment](https://github.com/SFTtech/openage/pull/69#issuecomment-61998049)) this improves the error message when an unknown enum value has been encountered. 

It simply suggests to `make media` :)

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
	---
```