Christopher Vourlias U2 frontman Bono made a surprise appearance Friday night at the opening ceremony of the Sarajevo Film Festival, with the Irish pop star leading a rapturous crowd in a cappella rendition of Bob Marley’s “Redemption Song.” The legendary vocalist, appearing alongside bandmate the Edge, took the stage after an emotional screening of “Kiss the Future,” director Nenad Cicin-Sain’s documentary, produced by Ben Affleck and Matt Damon, about U2’s relationship with war-torn Sarajevo in the 1990s. Based on American-born aid worker Bill Carter’s “Fools Rush in: A Memoir,” the film chronicles the band’s efforts to publicize the plight of the city’s besieged civilians during the Bosnian War.