CNO Financial Group’s top executives are a conservative lot who have spent much of the past decade stabilizing and reducing the risk profile of the Carmel-based insurer. So imagine their surprise when they got caught up in what prosecutors have described as perhaps the biggest fraud since Bernie Madoff. The saga began in late 2013 with a deal CNO struck to offload exposure to long-term-care insurance policies that had the potential to saddle the company with big losses down the road.