MDL-70282 oauth: Improve error information

If the request to the OAuth 2 token endpoint fails show the response
body the endpoint returned with its HTTP status (when debug: DEVELOPER).
If no response is available show any error returned by Curl.  Previously
none of this information was available making troubleshooting difficult.

If a token refresh fails in \core\oauth2\refresh_system_tokens_task an
exception is now thrown so that the result is shown as "Fail" on
admin/tasklogs.php?filter=core\oauth2\refresh_system_tokens_task

lib/classes/oauth2/client.php

@@ -403,7 +403,8 @@ class client extends \oauth2_client {
         }
 
         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);