Respond with a bad request if artifact path is invalid

lib/api/helpers.rb

@@ -222,7 +222,7 @@ module API
 
     def bad_request!(attribute)
       message = ["400 (Bad request)"]
-      message << "\"" + attribute.to_s + "\" not given"
+      message << "\"" + attribute.to_s + "\" not given" if attribute
       render_api_error!(message.join(' '), 400)
     end
 

lib/api/job_artifacts.rb

@@ -52,7 +52,7 @@ module API
 
         path = Gitlab::Ci::Build::Artifacts::Path
           .new(params[:artifact_path])
-        not_found! unless path.valid?
+        bad_request! unless path.valid?
 
         send_artifacts_entry(build, path)
       end