Elixir With special form

elixir_with.ex

case File.read("my_file.ex") do
  {:ok, contents} ->
    case Code.eval_string(contents) do
      {res, _binding} ->
        {:ok, res}
      error ->
        error
  error -> error
    error
end

with {:ok, contents} <- File.read("my_file.ex"),
     {res, binding} <- Code.eval_string(contents),
     do: {:ok, res}

elixir_with2.ex

# Create a customer and a promotional coupon
case Repo.insert(%Customer{name: "jose valim"}) do
  {:ok, customer} ->
    case Repo.insert(%Coupon{customer: customer, perc: 40}) do
      {:ok, coupon} -> Notify.success(customer)
      {:error, result} -> Notify.error(customer)
    end
  {:error, result} ->
    Notify.error(result)
end

# With
with {:ok, customer} <- Repo.insert(%Customer{name: "jose valim"})
     {:ok, coupon} <- Repo.insert(%Coupon{customer: customer, perc: 40})
     do: Notify.success(customer)

When you have the with keyword, how do you specify an error?