Add not found error

Component throw error if vue.js or sortable.js not loaded!

Author: y4roc

dist/vuedraggable.js

@@ -369,5 +369,13 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
   } else if (window && window.Vue && window.Sortable) {
     var draggable = buildDraggable(window.Sortable);
     Vue.component('draggable', draggable);
+  } else {
+    if(typeof window.Vue == "undefined") {
+      console.error('Vue.js not found!');
+    }
+    
+    if(typeof window.Sortable == "undefined") {
+      console.error('Sortable.js not found!');
+    }
   }
-})();
+})();