In any instance in which any tract of land not susceptible of irrigation in its natural state has been included in any general improvement district or divisional district herein authorized through inadvertency or mistake on the part of the district board at the time of the organization of such general improvement district or divisional district, the same may be excluded from the district concerned by a petition made by the owner or owners thereof and filed with the district board: PROVIDED, That the exclusion of said land or lands shall not relieve the same of its obligation to pay assessments for bonds outstanding at the time said petition is filed with the district board without written consent of the holders of said bonds.