unpacker: check the right exception type for 2.4

hgweb: fix diffstat links in paper/changeset.tmpl '<a .../>foo</a>' syntax is incorrect, since the first tag just "tries" to close itself and then the actual content follows. It doesn't work, either because web browsers know better than this or because there should be a whitespace before /: '<a />'. So for the hgweb users the links looked normal anyway, but now they are correct in code as well.

hgweb: close <img> elements Templates declare xhtml doctype, which means, in particular, that the document must also be valid xml. So <img> elements must be closed.