Before this change, such bookmarks are write-only: a client can push
them but not pull/read them. And because these bookmark can't be read,
even pushes are limited (for instance trying to delete such a bookmark
fails with a vanilla client because the client thinks the bookmark is
neither on the local nor the remote).
This change makes the server refuses such bookmarks, and for earlier
errors, makes the client refuse to send them.
I think the change of behavior is acceptable because I think this is a
bug in push/pull, and I don't think we change the behavior of `hg
unbundle`, because it doesn't seem that hg bundle ever store
bookmarks (and even if it did, it would seem weird anyway to try to
send divergent bookmarks).