在Umbraco XSLT宏中优雅地处理缺少的XML提要

I'm trying to read in an XML feed using an XSLT Macro in Umbraco and have it display the content nicely formatted. My Macro works fine when the feed is available, but if the feed returns a 404 I cannot manage to get the XSLT to gracefully handle it. I'm trying to read in an XML feed using an XSLT