(1) An action for breach of any contract for sale must be commenced within 6 years after the cause of action has accrued. By the original agreement the parties, if they are merchants, may reduce the period of limitation to not less than one year. The period of limitation may not otherwise be varied by agreement. (2) A cause of action accrues when the breach occurs, regardless of the aggrieved party's lack of knowledge of the breach. A breach of warranty occurs when tender of delivery is made, except