FIX: Return 422 when creating topics with tags w/out permission (#10400)

The UI prevents users from trying to create tags on topics when they
don't have permission, but if you are trying to add tags to a topic via
the API and you don't have permission before this change it would
silently succeed in creating the topic, but it wouldn't have any tags.

Now a 422 error will be returned with an error message when trying to
create a topic with tags when tagging is disabled or you don't have
enough trust level to add tags to a topic.

Bug report: https://meta.discourse.org/t/-/70525/14

lib/topic_creator.rb

@@ -165,7 +165,10 @@ class TopicCreator
       end
     else
       valid_tags = DiscourseTagging.tag_topic_by_names(topic, @guardian, @opts[:tags])
-      rollback_from_errors!(topic) unless valid_tags
+      unless valid_tags
+        topic.errors.add(:base, :unable_to_tag)
+        rollback_from_errors!(topic)
+      end
     end
   end