MDL-71119 core_badges: Add more information to errors

Some errors raised when calling external services were ignored.
Displaying this information helps to debug and find the real reasons
why some action, like connecting to an external backpack, can't be
done.

badges/classes/oauth2/client.php

@@ -245,11 +245,13 @@ class client extends \core\oauth2\client {
         }
         // Requests can either use http GET or POST.
         $response = $this->post($this->token_url(), $this->build_post_data($params));
-        $r = json_decode($response);
         if ($this->info['http_code'] !== 200) {
-            throw new moodle_exception('Could not upgrade oauth token');
+            $debuginfo = !empty($this->error) ? $this->error : $response;
+            throw new moodle_exception('oauth2refreshtokenerror', 'core_error', '', $this->info['http_code'], $debuginfo);
         }
 
+        $r = json_decode($response);
+
         if (is_null($r)) {
             throw new moodle_exception("Could not decode JSON token response");
         }