virt-viewer: Avoid simple_message_dialog() when errors can be propagated
    
    Remove all the dialogs used to report errors on extract_connect_info()
    and just propagate the errors we got from it.
    The only exception is virt_viewer_domain_event(), that is a callback
    that doesn't have GError as argument. In this specific case, we show the
    error dialog instead of propagating it.
    
    Related: rhbz#1085216