AO3-6791 Raise 404 when editing a pseud for a user that doesn't exist

app/controllers/pseuds_controller.rb

@@ -74,6 +74,8 @@ def new
 
   # GET /pseuds/1/edit
   def edit
+    raise ActiveRecord::RecordNotFound, "Couldn't find user '#{params[:user_id]}'" unless @user
+
     @pseud = @user.pseuds.find_by!(name: params[:id])
     authorize @pseud if logged_in_as_admin?
   end

spec/controllers/pseuds_controller_spec.rb

@@ -81,6 +81,11 @@
             expect { get :edit, params: { user_id: user, id: "fake_pseud" } }
               .to raise_error(ActiveRecord::RecordNotFound)
           end
+
+          it "returns NotFound error when user doesn't exist" do
+            expect { get :edit, params: { user_id: "fake_user", id: pseud } }
+              .to raise_error(ActiveRecord::RecordNotFound)
+          end
         end
       end
     end