Report a useful mixer id, rather than ""

This was D8614, which got a "ship it" 30 months ago (but not an
approval in Phab). The typo it incidentally fixed was fixed
by Yuri in 2018.

- Move warning about unregistered card down, so that the warning
  contains the newly-generated ID; otherwise the warning message is
  nonsensical, because it *hasn't* created a new ID yet.