If for any reason any part of the principal or interest of the bonds remains unpaid after enforcement of the assessment as provided in this division, the board shall order an additional or supplemental assessment to be made as provided in this division sufficient to pay the unpaid principal and interest.
Where bonds of the district have been authorized to be issued on the assessments, all unpaid assessments bear interest at the rate of 7 percent per year from the date of the bonds issued thereon until the bonds have been fully paid and discharged.