Desync issue between nbgrader and nbexchange

[lzach]

The current plugin only checks the exchange for the status of assignments/feedback and thus if the notebook has a manual change in it (such as deletion of an assignment folder), the plugin does not take this into account. This can lead to, among other things, it being impossible to redownload a deleted assignment (as the exchange has the assignment as already downloaded).

[ykazakov]

I cannot reproduce this problem. If I delete the downloaded assignment I can fetch it again from the assignment list menu.