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