Elizabeth Holmes once stood on a stage and said her company's blood test could see cancer coming 17 years before you ever got sick. She got that number from dead people. Back in 2014, at the Computer History Museum in Mountain View, she told the audience Theranos had worked with people at Johns Hopkins who showed you could see pancreatic cancer in blood "17 years before a tumor forms." There really was a Hopkins study with a number like that. It came out in Nature in 2010, and the way they…