After 'mix phx.gen.json Items Item items name:string description:string info:string amount:integer factor:float type:string --no-context --no-schema'.

lib/generic_rest_server_web/controllers/fallback_controller.ex

@@ -0,0 +1,16 @@
+defmodule GenericRestServerWeb.FallbackController do
+  @moduledoc """
+  Translates controller action results into valid `Plug.Conn` responses.
+
+  See `Phoenix.Controller.action_fallback/1` for more details.
+  """
+  use GenericRestServerWeb, :controller
+
+  # This clause is an example of how to handle resources that cannot be found.
+  def call(conn, {:error, :not_found}) do
+    conn
+    |> put_status(:not_found)
+    |> put_view(html: GenericRestServerWeb.ErrorHTML, json: GenericRestServerWeb.ErrorJSON)
+    |> render(:"404")
+  end
+end