Hmm interesting, changed it to add a onError handler but nothing was output. Not sure if I did it 100% right but this is what I changed.
head.onSuccess = function(head) {
meshFat = head.loadedMeshes[0];
console.log("No. of vertices for fat: "+meshFat.getTotalVertices());
}
head.onError = function (head, message, exception) {
console.log(head, message, exception);
}