File Upload: Improve message when user tries to upload duplicate file.

[kcondon]

See RT 225830
User attempted to upload a duplicate file with a different name to an existing set of files and became very frustrated because the app only said the file already exists but gave no other information.

I believe we used to provide more information. Maybe we can at least provide the duplicate file name and reason:

A file with the same content ,, already exists identified by MD5: .

[pdurbin]

@raprasad @landreev @sekmiller @scolapasta has the behavior changed in the 2290-file-replace branch? Is the GUI and the API consistent? I'm moving this to the dev column in Waffle so we can at least talk about it. It's highly related to #2290.