Bonds issued under the provisions of this chapter shall not be deemed to constitute a pledge of the faith and credit of the Commonwealth or of any governmental unit thereof. All such bonds shall contain a statement on their face substantially to the effect that neither the faith and credit of the Commonwealth nor the faith and credit of any governmental unit of the Commonwealth is pledged to the payment of the principal of or the interest on such bonds. The issuance of bonds under the provisions of this chapter shall not directly, indirectly or contingently obligate the Commonwealth or any governmental unit of the Commonwealth to levy any taxes whatever therefor or to make any appropriation for their payment.