Found by accidentally publishing {urn:xmpp:bookmarks:0}conference
instead of :1 due to testing this earlier for the blocking.
By the principle of garbage in, garbage out, just generate a bookmark
from the item id / JID and carry on with a warning.
Having both the :0 and :1 nodes would be especially awkward, since there
is no upgrade path for this case. In theory, these should be rare since
no clients should have been doing XEP-0402 unless mod_bookmarks(2) was
enabled. This was guesstimated to be rare with most clients doing
XEP-0048 with Private XML.
