fix error message duplication for duplicate filename ( fixes #1309 )

[MontrealSergiy]

Fixes confusing error messages when trying to upload a file that already exists shows bad error message

I did not investigate messages for any other errors.

https://github.com/aces/cbrain/issues/1309

[prioux]

I doesn't seem to work. I tried uploading twice the same file and the second time I still get the ugly 422 Unprocesseable Entity message.

[prioux]

Never mind, it works, my Firefox was still caching the old javascript.