that server has no events to send. Only <code>onComplete</code> is invoked.</li>
 <li>503 - with <code><a href="../core/HttpHeaders.html#RETRY_AFTER">"Retry-After"</a></code> header set to a valid value: This indicates
 that the server is unavailable, but that the client should reconnect later. No consumers are invoked unless the client
 event source is closed, prior to reconnecting (resulting in <code>onComplete</code> invocation). After the specified
 delay, the client should automatically attempt to reco