In case any taxing district is unable to sell its certificates of indebtedness, it may issue warrants in payment of current expenses in excess of cash on hand, but not in excess of eighty-five percent of taxes levied for the fiscal year of issue but uncollected and not otherwise encumbered, plus fifty percent of the uncollected and not otherwise encumbered taxes of the four preceding years, and the funds derived from the collection of taxes for the current year and such preceding years, to the extent that the same have been encumbered, constitute a special fund for the payment of warrants issued against such taxes. If warrants are issued in excess of     such limitations, such warrants possess no validity as against the taxing district, but the officials knowingly and willfully issuing the same are liable personally for the payment thereof.