Tales of overcoming adversity have long been the stuff of storytellers, from the days of the ancients to the present. Their stories have enlightened legions of listeners and, more recently, in the age of cinema, viewers. Such sagas have profoundly moved audiences, filling them with inspiration, hope and the courage to carry forth under trying conditions. When well told, those stories prove incredibly effective. But, when their recounting doesn’t live up to their subject matter, the result is disappointment, as is the case with the new historical biography, “12 Years a Slave,” based on the memoir of the same name.