VATICAN CITY — Pope Francis told leading oil executives Saturday that the transition to less-polluting energy sources “is a challenge of epochal proportions” and warned that satisfying the world’s energy needs “must not destroy civilization.” The Vatican said Francis held a two-day conference with the executives as a follow-up to his encyclical three years ago that called on people to save the planet from climate change and other environmental ills. Participants included the CEOs of Italian oil giant ENI, British Petroleum, ExxonMobil, Norway’s Statoil , scientists and managers of major investment funds.