errors: move revlog errors
- create error.py for exception classes to reduce demandloading
- move revlog exceptions to it
- change users to import error and drop revlog import if possible
<item>
<title>#desc|strip|firstline|strip|escape#</title>
<link>{urlbase}{url}log{#node|short#}/{file|urlescape}</link>
<description><![CDATA[#desc|strip|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>