Display user friendly message when adding file with the same name but different extension to Guvnor Repository

It's not possible to add file with the same name and different extension to Guvnor Repository. When trying to do it standard error message saying nothing relevant is displayed. See attached screenshot.

We should display more descriptive message. Something like "Unable to add <file> to Guvnor Repository because file with the same name and different extension already exists in repository (<filefromrep>). No file was added to Guvnor repository".