Handle a nonexistent repository with a 404 error.
Based on changeset 62ea868cafbfa3acb7e5dba6982ecb46a1637246 from
Vincent Wagelaar <vincent@ricardis.tudelft.nl>
in http://hannibal.lr-s.tudelft.nl/~vincent/fcgi
default = "changelog"
header = header-rss.tmpl
changelog = changelog-rss.tmpl
changelogentry = changelogentry-rss.tmpl
filelog = filelog-rss.tmpl
filelogentry = filelogentry-rss.tmpl