Switch out FILE_ALREADY_EXISTS for DUPLICATE_ITEM.

Review Request #8137 — Created April 30, 2016 and submitted — Latest diff uploaded

Information

Review Board
master
1d0ebb1...

Reviewers

The original image dnd branch included a new API error for when a file
attachment was a duplicate, but we now have a generic error in djblets for
cases like this. This change swaps out the new error for the existing one.

Ran unit tests.