Hi,
It seems that if (for example) you mistakenly try and add target_usernames to a post with user_ids rather than usernames, create_post in scripts/import_scripts/base.rb won’t return an error.
This is because it returns: post ? post : post_creator.errors.full_messages. However, post has already been initialised as a Post type on line 564. Checking for the post_creator errors instead returns the correct behaviour.
I’ll submit a pull request for this 
