The official acts of any person in possession of a public office and exercising the functions thereof shall be valid and binding as official acts in regard to all persons interested or affected thereby, whether such person be lawfully entitled to hold the office or not and whether such person be lawfully qualified or not; but such person shall be liable to all the penalties imposed by law for usurping or unlawfully holding office, or for exercising the functions thereof without lawful right or without being qualified according to law.