All debts, contracts and obligations heretofore made or incurred by or in favor of any water district and all bonds, warrants, or other obligations issued by such district, and all charges heretofore made by such districts, and any and all assessments heretofore levied in any local improvement districts or utility local improvement districts of any water district, and all other things and proceedings relating thereto done or taken by such water districts or by their respective officers are hereby declared to be legal and valid and of full force and effect from the date thereof: PROVIDED, That nothing in this section shall apply to ultra vires acts or acts of fraud committed by the officers or agents of said district.