UX: Improve error message when a topic cannot be moved due to category restrictions (#20900)

app/controllers/topics_controller.rb

@@ -361,7 +361,14 @@ class TopicsController < ApplicationController
         category = Category.find_by(id: params[:category_id])
 
         if category || (params[:category_id].to_i == 0)
-          guardian.ensure_can_move_topic_to_category!(category)
+          begin
+            guardian.ensure_can_move_topic_to_category!(category)
+          rescue Discourse::InvalidAccess
+            return(
+              render_json_error I18n.t("category.errors.move_topic_to_category_disallowed"),
+                                status: :forbidden
+            )
+          end
         else
           return render_json_error(I18n.t("category.errors.not_found"))
         end

config/locales/server.en.yml

@@ -705,6 +705,7 @@ en:
       disallowed_tags_generic: "This topic has disallowed tags."
       slug_contains_non_ascii_chars: "contains non-ascii characters"
       is_already_in_use: "is already in use"
+      move_topic_to_category_disallowed: "You cannot move this topic to a category where you do not have permission to create new topics."
     cannot_delete:
       uncategorized: "This category is special. It is intended as a holding area for topics that have no category; it cannot be deleted."
       has_subcategories: "Can't delete this category because it has sub-categories."

spec/requests/topics_controller_spec.rb

@@ -1391,6 +1391,9 @@ RSpec.describe TopicsController do
         put "/t/#{topic.id}.json", params: { category_id: category.id }
 
         expect(response.status).to eq(403)
+        expect(response.parsed_body["errors"].first).to eq(
+          I18n.t("category.errors.move_topic_to_category_disallowed"),
+        )
         expect(topic.reload.category_id).not_to eq(category.id)
       end