[Mines] Drop in ease of mining during running out

[Thomas8]

Rather than immedietaly running out of resources and destroying asset - it would be good for mine to drop 'ease of mining' level (by 2 or 3 points) when it runs out of resources and refill it with (maybe?) with half of original value. Only destroy mine once you drop below 'zero level'.

That way you wont run out of resources by suprise - and can slowly transition to more expensive alternative. This will also make 'ease of mining' actually significant - right now you can simply upgrade mine and dont worry about lower digging ratio for rest of the game.