Just give regular 404, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5094#note_12984211

app/controllers/projects/artifacts_controller.rb

@@ -25,7 +25,7 @@ class Projects::ArtifactsController < Projects::ApplicationController
     if entry.exists?
       send_artifacts_entry(build, entry)
     else
-      render json: {}, status: 404
+      render_404
     end
   end