explain redirect back error, when it comes
Rails API (api.rubyonrails.org) for redirect_to says:
When using redirect_to :back, if there is no referrer, RedirectBackError will be raised. You may specify some fallback behavior for this case by rescueing RedirectBackError.
The Referer header is usually sent with a request by a browser if the user goes to the page by clicking a link, and it contains the url of the page with that link. There is no Referer header if the user just types the address in the address bar, for example.