MDL-65518 badges: Admin settings warning

If the backpack is changed to Open Badges V2 backpack,
attempt the authenticate with the site settings and return a warning
if it fails.

badges/classes/output/external_backpacks_page.php

@@ -70,6 +70,7 @@ class external_backpacks_page implements \renderable {
             }
             $data->backpacks[] = $backpack;
         }
+        $data->warning = badges_verify_site_backpack();
 
         return $data;
     }

badges/templates/external_backpacks_page.mustache

@@ -26,7 +26,8 @@
     {
         "backpacks": [
             {"backpackweburl": "http://localhost/", "sitebackpack": true, "canedit": false}
-        ]
+        ],
+        "warning": "<span class='text-warning'>Could not login</span>"
     }
 }}
 <table class="generaltable fullwidth">
@@ -54,3 +55,4 @@
         {{/backpacks}}
     </tbody>
 </table>
+{{{warning}}}