He does not punish Moses and does not give excuses or an explanation. In his patience, God allows Moses to question him and voice his objections.
God is not a dictator who does not tolerate contradiction, but he is a Father full of patience and understanding. And then he repeats his promise. In the continuation (starting in Exodus 6:2), he sends Moses back to work.
We read more often in the Bible that prophets lose hope and complain: Elijah in 1 Kings 19:1–21 and Jeremiah in Jeremiah 20:14–18. God patiently lets them finish what they are saying, and then sends them back to work.
1 L'Eternel dit à Moïse: Tu verras maintenant ce que je ferai à Pharaon; une main puissante le forcera à les laisser aller, une main puissante le forcera à les chasser de son pays.