FIX: better error message when topic deletion fails

app/controllers/topics_controller.rb

@@ -557,6 +557,8 @@ class TopicsController < ApplicationController
     PostDestroyer.new(current_user, first_post, context: params[:context]).destroy
 
     render body: nil
+  rescue Discourse::InvalidAccess
+    render_json_error I18n.t("delete_topic_failed")
   end
 
   def recover

config/locales/server.en.yml

@@ -266,6 +266,7 @@ en:
     request_membership: "Request Membership"
     join_group: "Join Group"
   deleted_topic: "Oops! This topic has been deleted and is no longer available."
+  delete_topic_failed: "There was an error deleting that topic. Please contact the site's administrator."
 
   reading_time: "Reading time"
   likes: "Likes"

spec/requests/topics_controller_spec.rb

@@ -860,7 +860,7 @@ RSpec.describe TopicsController do
         it "raises an exception when the user doesn't have permission to delete the topic" do
           sign_in(user)
           delete "/t/#{topic.id}.json"
-          expect(response).to be_forbidden
+          expect(response.status).to eq(422)
         end
       end