The logic to correctly associate messages where the parent message is not in
the archives already existed by means of `ThreadTree::$extraRootChildren`,
however the “extra root” threads were not correctly printed, because
`printThread()` required the message at the root of the thread to exist.
Fix this by printing a placeholder “Unknown message” for unknown messageIds and
then printing the children as usual.
Fixesphp/web-php#35