Add better crash message for zero-length battlefield list

lib/gameState/CGameState.cpp

@@ -1099,6 +1099,9 @@ BattleField CGameState::battleGetBattlefieldType(int3 tile, CRandomGenerator & r
 	if(map->isCoastalTile(tile)) //coastal tile is always ground
 		return BattleField(*VLC->identifiers()->getIdentifier("core", "battlefield.sand_shore"));
 	
+	if (t.terType->battleFields.empty())
+		throw std::runtime_error("Failed to find battlefield for terrain " + t.terType->getJsonKey());
+
 	return BattleField(*RandomGeneratorUtil::nextItem(t.terType->battleFields, rand));
 }