Redirect to 404 page if css_id in URL is invalid (#14172)

* redirect to our normal 404 page if css_id is invalid
* update test

app/controllers/legacy_tasks_controller.rb

@@ -126,7 +126,7 @@ def update
   private
 
   def validate_user_id
-    fail(Caseflow::Error::InvalidUserId, user_id: params[:user_id]) unless user
+    fail(ActiveRecord::RecordNotFound, user_id: params[:user_id]) unless user
   end
 
   def validate_user_role

spec/controllers/legacy_tasks_controller_spec.rb

@@ -73,10 +73,10 @@
       end
     end
     context "CSS_ID in URL is invalid" do
-      it "returns 400" do
+      it "returns 404" do
         [-1, "BAD_CSS_ID", ""].each do |user_id_path|
           get :index, params: { user_id: user_id_path, rest: "/assign" }
-          expect(response.status).to eq 400
+          expect(response.status).to eq 404
         end
       end
     end