Fix regression when showing disconnect error
    
    Commit 65ef66e42 introduced a regression due to lack of type-safety on
    signals. We mistakenly passed a GError rather than a string error
    message to the signal.
    
    Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>