Comment 3 for bug 1490508

There has been quite a few support requests from users experiencing the "object has no attribute 'slidenumber'" traceback, so we should probably try to fix this. As I see it there are (at least) 2 things that could cause this:
1. Loading of the presentation failed, so PowerpointDocument wasen't initilized properly
2. The presentation was loaded by Impress, creating an ImpressDocument, which causes issues when trying to start this with PowerPoint.