dirstate: stringify a few exception messages
Built in exceptions want str, and ProgrammingError converts bytes to str
internally (because it subclasses RuntimeError).
<item>
<title>{bookmark|escape}</title>
<link>{urlbase}{url|urlescape}rev/{node|short}</link>
<description><![CDATA[{bookmark|strip|escape|addbreaks}]]></description>
<pubDate>{date|rfc822date}</pubDate>
</item>