Neither the final order of determination nor the final order of apportionment shall be subject to attack in any court except by proceedings in certiorari brought within 30 days after the filing of such order in the office of the secretary of the board issuing the same. If no such proceeding shall be brought within the time above prescribed, the project shall be deemed to have been legally established and the legality of the project and the assessments therefor shall not thereafter be questioned in any suit at law or in equity: Provided, That all notices given under this chapter shall include the language contained in this section prior to this provision.