After "mix phx.gen.json BiddingRounds BiddingRound bidding_rounds round_number:integer running:boolean --no-scope".

lib/beet_round_server_web/controllers/fallback_controller.ex

@@ -0,0 +1,24 @@
+defmodule BeetRoundServerWeb.FallbackController do
+  @moduledoc """
+  Translates controller action results into valid `Plug.Conn` responses.
+
+  See `Phoenix.Controller.action_fallback/1` for more details.
+  """
+  use BeetRoundServerWeb, :controller
+
+  # This clause handles errors returned by Ecto's insert/update/delete.
+  def call(conn, {:error, %Ecto.Changeset{} = changeset}) do
+    conn
+    |> put_status(:unprocessable_entity)
+    |> put_view(json: BeetRoundServerWeb.ChangesetJSON)
+    |> render(:error, changeset: changeset)
+  end
+
+  # 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: BeetRoundServerWeb.ErrorHTML, json: BeetRoundServerWeb.ErrorJSON)
+    |> render(:"404")
+  end
+end