Steps to reproduce:-->
1.Go to Documents in The content perspective.
2.Select any document with image link in it.
3.Upload a new image using the image picker either by clicking on image link or in the ckeditor image link button
4.Try to upload the same image for the 2ndtime.
5."Action failed" message is displayed on UI.
Expected:-->Clear error should be shown to user that this image is already uploaded.