Raise a more informative error when `request.env['warden']` is `nil`.
Raise a more informative error when `request.env['warden']` is `nil`.

Previously, a `NoMethodError` exception would be raised from here when the
middleware stack isn't present and Warden wasn't injected as expected
(like in a controller test). To foolproof ourselves, we now raise a more
informative error when `request.env['warden']` is `nil` so developers can
figure this out on their own instead of reaching to the issue tracker for
guidance.

rubyrider/devise,fjg/devise,rubyrider/devise,Qwiklabs/devise,stanhu/devise,ngpestelos/devise,Qwiklabs/devise,plataformatec/devise,ngpestelos/devise,stanhu/devise,fjg/devise,plataformatec/devise,rubyrider/devise