FIX: show proper error message when no user selected for private message

lib/has_errors.rb

@@ -13,7 +13,7 @@ module HasErrors
   end
 
   def rollback_with!(obj, error)
-    obj.errors[:base] << error
+    obj.errors.add(:base, error)
     rollback_from_errors!(obj)
   end
 

lib/post_creator.rb

@@ -317,8 +317,7 @@ class PostCreator
       topic_creator = TopicCreator.new(@user, guardian, @opts)
       @topic = topic_creator.create
     rescue ActiveRecord::Rollback
-      add_errors_from(topic_creator)
-      return
+      rollback_from_errors!(topic_creator)
     end
     @post.topic_id = @topic.id
     @post.topic = @topic

spec/components/has_errors_spec.rb

@@ -45,10 +45,10 @@ describe HasErrors do
     it "triggers a rollback" do
 
       expect(-> {
-        error_test.rollback_with!(invalid_topic, :custom_error)
+        error_test.rollback_with!(invalid_topic, :too_many_users)
       }).to raise_error(ActiveRecord::Rollback)
       expect(error_test.errors).to be_present
-      expect(error_test.errors[:base]).to include(:custom_error)
+      expect(error_test.errors[:base]).to include("You can only send warnings to one user at a time.")
     end
   end