Update return values on Creature events. (otland#2171)

Those functions are expecting a ReturnValue as return not a boolean, this might confuse someone.

data/events/scripts/creature.lua

@@ -3,9 +3,9 @@ function Creature:onChangeOutfit(outfit)
 end
 
 function Creature:onAreaCombat(tile, isAggressive)
-	return true
+	return RETURNVALUE_NOERROR
 end
 
 function Creature:onTargetCombat(target)
-	return true
+	return RETURNVALUE_NOERROR
 end