Fix action do not provide icon.

action might be undefined then button will get no icon.
Prevent throwing an uncatched error and insert warning sign on button

Will do better one I allow text instead of icon

IPython/html/static/notebook/js/toolbar.js

@@ -124,7 +124,7 @@ define([
                     .addClass('btn btn-default')
                     .attr("title", el.label||action.help)
                     .append(
-                        $("<i/>").addClass(el.icon||action.icon).addClass('fa')
+                        $("<i/>").addClass(el.icon||(action||{icon:'fa-exclamation-triangle'}).icon).addClass('fa')
                     );
                 var id = el.id;
                 if( id !== undefined ){