Rapper Eminem has cancelled his forthcoming European tour, blaming exhaustion.    He was due to play ten concerts alongside 50 Cent and D-12 next month in Britain, France, Germany, Denmark, Sweden and the Netherlands. But the tired star has now called off the European dates of his Anger Management 3 tour and is unlikely to reschedule them.   The US leg wrapped up in his native Detroit, Michigan, on Saturday. A statement from his record company Interscope says, "Eminem is currently being treated for exhaustion, complicated by other medical issues." Last month, Eminem slammed reports he planned to retire, although he confirmed his intention to take a break. He now faces rumours that he is addicted to sleeping pills, with his publicist stating that he "is in the hospital under doctors' care".