SANAA, Yemen — An airstrike by the Saudi-led coalition hit a wedding party in northern Yemen, killing at least 20 people, health officials said Monday, as harrowing images emerged on social media of the deadly bombing, the third to hit Yemeni civilians since the weekend. Khaled al-Nadhri, the top health official in the northern province of Hajja, told The Associated Press that most of the dead were women and children who were gathered in one of the tents set up for the wedding party in the Bani Qayis district.