Set flash error when unsuccessfully destroying resource

Prior to this commit, the user would not get any error message when
trying to destroy a resource using HTML instead of JS. This fixes that.

It also uses the `render_js_for_destroy` method to render the success
flash.

backend/app/controllers/spree/admin/resource_controller.rb

@@ -99,16 +99,27 @@ def destroy
 
     if destroy_result
       invoke_callbacks(:destroy, :after)
-      flash[:success] = flash_message_for(@object, :successfully_removed)
       respond_with(@object) do |format|
-        format.html { redirect_to location_after_destroy }
-        format.js   { render partial: "spree/admin/shared/destroy" }
+        message = flash_message_for(@object, :successfully_removed)
+        format.html do
+          flash[:success] = message
+          redirect_to location_after_destroy
+        end
+        format.js do
+          render_js_for_destroy
+        end
       end
     else
       invoke_callbacks(:destroy, :fails)
       respond_with(@object) do |format|
-        format.html { redirect_to location_after_destroy }
-        format.js { render status: :unprocessable_entity, plain: @object.errors.full_messages.to_sentence }
+        message = @object.errors.full_messages.to_sentence
+        format.html do
+          flash[:error] = message
+          redirect_to location_after_destroy
+        end
+        format.js do
+          render status: :unprocessable_entity, plain: message
+        end
       end
     end
   end

backend/spec/controllers/spree/admin/resource_controller_spec.rb

@@ -168,6 +168,16 @@ def check_destroy_constraints
           expect(response.body).to eq assigns(:widget).errors.full_messages.to_sentence
         end
       end
+
+      context 'html format' do
+        subject { delete :destroy, params: params }
+
+        it 'responds with error message' do
+          subject
+          expect(response).to be_redirect
+          expect(flash[:error]).to eq assigns(:widget).errors.full_messages.to_sentence
+        end
+      end
     end
   end