At least log an error if there is no create or remove defined.

We might just remove this...

IPython/html/static/widgets/js/widget.js

@@ -621,6 +621,9 b' define(["widgets/js/manager",'
             // returns a promise that resolves after this update is done
             var remove = remove_view || this._remove_view;
             var create = create_view || this._create_view;
+            if (create === undefined || remove === undefined){
+                console.error("Must define a create a remove function");
+            }
             var context = context || this._handler_context;
             var added_views = [];
             var that = this;