Instead of dhtmlxAjax you should use dhx.ajax for ajax operations. See dhtmlxAjax API for details.
Instead of dhtmlxError.catchError you should use onLoadXMLError handler.
Similar to doXpath in older version of API you can use xmltop and xpath methods
var root = dhx.ajax.xmltop("rows", xml_object);
var nodes = dhx.ajax.xpath(xpath_expression, root_node);