On some source .html pages, po4a wrongly detects a malformed document,
which makes the po plugin error out and the wiki build is aborted.

I've pushed a [[patch]] to my `po` branch to fix this: it makes po4a
warn, instead of error'ing out, when it detects a malformed input
document.

This is really a po4a bug which I will report, but since most ikiwiki
users are gonna live with Squeeze's po4a for a while, I think we
should workaround it in ikiwiki.

Also, the current state of things makes it a bit too easy to break a
given ikiwiki site build (DoS) when both the html and po plugins are
enabled: inserting a html tag without closing it is enough.

--[[intrigeri]]