The want of performance of an obligation, or of an offer of performance, in whole or in part, or any delay therein, is excused by the following causes, to the extent to which they operate: 1. When such performance or offer is prevented or delayed by the act of the creditor, or by the operation of law, even though there may have been a stipulation that this shall not be an excuse; however, the parties may expressly require in a contract that the party relying on the provisions of this paragraph give