What Was Luke's "Failure At the Cave"?

While watching a Clone Wars episode where Yoda visits the same dark side cave Luke did in The Empire Strikes Back, a thought struck me: what did Yoda mean when he reminded Luke of his "failure at the cave"? We all remember the slightly trippy scene in which young Skywalker faced a phantom Vader, only to see his own face in the Dark Lord's mask. Join me as we discuss a line of dialogue that suggests Luke "failed" at this encounter.