I have a login page which allows to authenticate an admin.
If there are no users at all in the database (first time the app runs), I will create the superuser based on the login data entered (email/password).

My login form looks like this:

<h1>Login</h1>

<%= form_for @conn,
             Routes.admin_session_path(@conn, :create),
             [as: :session],
             fn f -> %>
  <div>
    <%= text_input f, :email, placeholder: "Email" %>
    <%= error_tag f, :email %>
  </div>

  <div>
    <%= password_input f, :password, placeholder: "Password" %>
    <%= error_tag f, :password %>
  </div>

  <%= submit "Log in" %>
<% end %>

And my controller actions:

def new(conn, _params) do
  render(conn, "new.html")
end

def create(conn, %{"session" => %{"email" => email, "password" => pass}}) do
  case Accounts.authenticate_by_email_and_pass(email, pass) do
    {:ok, %{"active" => true} = user} ->
      conn
      |> MyAppWeb.Authentication.login(user)
      |> put_flash(:info, "Welcome back!")
      |> redirect(to: Routes.admin_question_path(conn, :index))

    {:ok, %{"active" => false}} ->
      conn
      |> put_flash(:error, "Account is disabled")
      |> render("new.html")

    {:error, _reason} ->
      case Accounts.count_users() do
        0 ->
          case Accounts.register_user(%{"name" => "Superuser", "email" => email, "password" => pass, "type" => "admin"}) do
            {:ok, user} ->
              conn
              |> put_flash(:info, "#{user.name} created!")
              |> redirect(to: Routes.admin_session_path(conn, :new))

            {:error, %Ecto.Changeset{} = changeset} ->
              conn
              |> put_flash(:error, changeset.errors)
              |> render("new.html")
          end
        _ ->
          conn
          |> put_flash(:error, "Invalid username/password combination")
          |> render("new.html")
      end
  end
end

Of course as expected I have a problem on this line:
put_flash(:error, changeset.errors)

But as the login form is not displayed based on a changeset (but rather uses the connection: form_for @conn), how do I display the list of changeset errors when the superuser is being attempted to be created?