[GUI] show error dialogs after the wizard has closed

Because it just looks ugly to have a dialog on top of another (wizard)
dialog which will automatically close if you close the error dialog and
so just waste user space.

Change-Id: Idee07a6144ee9eb7c8cec14f5464b4ee831a024d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1200
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>