Exceptions from liability under sections 153 and 154.
155. No liability shall be incurred by the owner of a ship under section 153 or 154 by reason of any discharge or escape of oil from the ship, or by reason of any relevant threat of contamination, if he proves that the discharge or escape, or (as the case may be) the threat of contamination—
(a)resulted from an act of war, hostilities, civil war, insurrection or an exceptional, inevitable and irresistible natural phenomenon; or
(b)was due wholly to anything done or omitted to be done by another person, not being a servant or agent of the owner, with intent to do damage; or
(c)was due wholly to the negligence or wrongful act of a government or other authority in exercising its function of maintaining lights or other navigational aids for the maintenance of which it was responsible.