manager: explicitly disallow login screen from opening reauth channel

It doesn't make sense for it to do, and right now the shell does it
up front, waits for the failure, and then does the "right" thing
(opens a new auth session) after.

This commit makes the failure explicit, so we can subsequently make
other cases where a reauth channel is requested work even if there is
no session to channel to by implicitly creating a transient one just
in time.  That will come later.

https://bugzilla.gnome.org/show_bug.cgi?id=726283