The board of supervisors shall canvass the returns and declare the result of the election. If it is found by the board of supervisors that a majority of the qualified electors of the county voting thereon assents to the issuance of the bonds, the board of supervisors shall provide for the issuance of the bonds. The determination of the board of supervisors that a majority of the qualified electors voting thereon has assented to the issuance of the bonds shall be conclusive in any action or proceeding involving the validity of such election or the determination or declaration of the result thereof instituted after the date of the payment for and delivery of the bonds.