Warn the user if a save operation is aborted
It's possible to try to save during autosave or try to export when a save is going on, and in that case the savingMutex is locked and we'd cancel() the saving operation, silently, as if the user had pressed cancel in the export dialog.
Now we can return not just a bool, but succes, failure or busy from initiateSaving depending on whether the the document was busy, saving succeeded or failed.