The owner of an old Easthampton mill building is facing sticker shock as he tries to renovate the space.  Because of an error from Columbia Gas, James Wittmer as billed at only half of his usage since he bought the building three years ago.  Now the gas company wants $27,000 in retroactive payments.  Wittmer says he can't just pass the cost on to his new tenants, but he'll make an extra effort to fill the studio spaces to generate revenue.