an unrecoverable error occurs during processing.</li>
 <li>200 - with unsupported or missing <code><a href="../core/HttpHeaders.html#CONTENT_TYPE">"Content-Type"</a></code> header: This is an
 error condition. <code>onError</code> is invoked.</li>
 <li>204 - This indicates 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