“DNA offers the possibility of certainty in a few kinds of crimes — like rape, or perhaps a murder when the criminal struggled with the victim and left behind skin or blood evidence that can be tested for genetic material. But the flaws that exonerations have revealed are common to any number of criminal prosecutions: mistaken eyewitnesses, forensic experts who draw the wrong conclusions, jailhouse informants, hair and blood analysis, misguided arson investigations, fingerprint analysis.