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Registers the plug to call as a fallback to the controller action.

A fallback plug is useful to translate common domain data structures
into a valid `%Plug.Conn{}` response. If the controller action fails to
return a `%Plug.Conn{}`, the provided plug will be called and receive
the controller's `%Plug.Conn{}` as it was before the action was invoked
along with the value returned from the controller action.

## Examples

    defmodule MyController do
      use Phoenix.Controller

      action_fallback MyFallbackController

      def show(conn, %{"id" => id}, current_user) do
        with {:ok, post} <- Blog.fetch_post(id),
             :ok <- Authorizer.authorize(current_user, :view, post) do

          render(conn, "show.json", post: post)

In the above example, `with` is used to match only a successful
post fetch, followed by valid authorization for the current user.
In the event either of those fail to match, `with` will not invoke
the render block and instead return the unmatched value. In this case,
imagine `Blog.fetch_post/2` returned `{:error, :not_found}` or
`Authorizer.authorize/3` returned `{:error, :unauthorized}`. For cases
where these data structures serve as return values across multiple
boundaries in our domain, a single fallback module can be used to
translate the value into a valid response. For example, you could
write the following fallback controller to handle the above values:

    defmodule MyFallbackController do
      use Phoenix.Controller

      def call(conn, {:error, :not_found}) do
        |> put_status(:not_found)
        |> put_view(MyErrorView)
        |> render(:"404")

      def call(conn, {:error, :unauthorized}) do
        |> put_status(403)
        |> put_view(MyErrorView)
        |> render(:"403")
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