Tracing layer: add additional error reporting to active node promise

Users reported seeing an error with retrievingthe active node from the tracing
layer. The root of this problem is still unclear, but this additional error
check may help to discover it.

django/applications/catmaid/static/js/widgets/overlay.js

@@ -183,7 +183,11 @@ SkeletonAnnotations.atn.set = function(node, stack_viewer_id) {
 SkeletonAnnotations.atn.promise = function()
 {
   var overlay = SkeletonAnnotations.getTracingOverlay(this.stack_viewer_id);
-  var nodePromise = overlay.promiseNode(overlay.nodes[this.id]);
+  var node = overlay.nodes[this.id];
+  if (!node) {
+    return Promise.reject("Couldn't find node " + this.id + " in tracing layer");
+  }
+  var nodePromise = overlay.promiseNode(node);
   var isNewSkeleton = !this.skeleton_id;
 
   return nodePromise.then((function(result) {