Fixed assets urls in 500.html

public/views/500.html

@@ -7,8 +7,8 @@
 
     <title>Grafana</title>
 
-    <link rel="stylesheet" href="[[.AppSubUrl]]/css/grafana.dark.min.css" title="Dark">
-    <link rel="icon" type="image/png" href="[[.AppSubUrl]]/img/fav32.png">
+    <link rel="stylesheet" href="[[.AppSubUrl]]/public/css/grafana.dark.min.css" title="Dark">
+    <link rel="icon" type="image/png" href="[[.AppSubUrl]]/public/img/fav32.png">
 
   </head>