Fix 404 rendering, closes #1826

config/config.exs

@@ -5,7 +5,8 @@ config :livebook, LivebookWeb.Endpoint,
   url: [host: "localhost", path: "/"],
   pubsub_server: Livebook.PubSub,
   live_view: [signing_salt: "livebook"],
-  drainer: [shutdown: 1000]
+  drainer: [shutdown: 1000],
+  render_errors: [formats: [html: LivebookWeb.ErrorHTML], layout: false]
 
 # Configures Elixir's Logger
 config :logger, :console,

test/livebook_web/controllers/errors_test.exs

@@ -0,0 +1,12 @@
+defmodule LivebookWeb.ErrorsTest do
+  use LivebookWeb.ConnCase, async: true
+
+  test "renders 404", %{conn: conn} do
+    response = assert_error_sent :not_found, fn ->
+      get(conn, "/this/does/not/exist")
+    end
+
+    assert {404, _, body} = response
+    assert body =~ "No Numbats here!"
+  end
+end