FIX: Show a json api response when deleting a user with posts

A 500 error was actually caused with no response when using the api, so
it wasn't very clear that you need to delete the posts first when using
the api.

app/controllers/admin/users_controller.rb

@@ -379,7 +379,10 @@ class Admin::UsersController < Admin::AdminController
           }
         end
       rescue UserDestroyer::PostsExistError
-        raise Discourse::InvalidAccess.new("User #{user.username} has #{user.post_count} posts, so can't be deleted.")
+        render json: {
+          deleted: false,
+          message: "User #{user.username} has #{user.post_count} posts, so they can't be deleted."
+        }
       end
     end
   end

spec/controllers/admin/users_controller_spec.rb

@@ -529,9 +529,11 @@ describe Admin::UsersController do
           _post = create_post(topic: topic, user: delete_me)
         end
 
-        it "returns an error" do
+        it "returns an api response that the user can't be deleted because it has posts" do
           delete :destroy, params: { id: delete_me.id }, format: :json
-          expect(response).to be_forbidden
+          expect(response).to be_success
+          json = ::JSON.parse(response.body)
+          expect(json['deleted']).to eq(false)
         end
 
         it "doesn't return an error if delete_posts == true" do