A default error handler that can be used for failed authentication
already_authenticated(Plug.Conn.t, map) :: Plug.Conn.t
no_resource(Plug.Conn.t, map) :: Plug.Conn.t
unauthenticated(Plug.Conn.t, map) :: Plug.Conn.t
unauthorized(Plug.Conn.t, map) :: Plug.Conn.t