[Pope Francis arrives on the Greek Island of Lesbos]By Philip Pullella and Karolina Tagaris LESBOS, Greece (Reuters) - Pope Francis arrived on the Greek island of Lesbos on Saturday, turning the world's attention to the frontline of Europe's migrant crisis which has claimed hundreds of lives in the past year. Francis, leader of the world's 1.2 billion Roman Catholics, was scheduled to spend about six hours on the small Aegean island. "This is a trip that is a bit different than the others ... this is a trip marked by sadness," Francis told reporters on the aeroplane taking him to Lesbos.