Move the use of KMessageBox out of TodoModel

Add an error signal for the previous use instead. Using a widget message
box isn't necessarily the desired way to communicate errors for all
applications. This matters particularly for QML-based apps like Kalendar.
3 jobs for !60 with work/vkrause/move-messagebox-out-of-todomodel in 3 minutes and 37 seconds (queued for 3 seconds)
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