Add Repo name to timeout errors

Start with PR 332 and the supplied DbConnectionErrorTest reproduction. Run the two ExUnit cases to compare timeout and ownership errors, then document the difference and the missing Repo name without assuming a timeout implementation change. Done means the reproduction and observed behavior are recorded clearly.

In https://github.com/elixir-ecto/db_connection/pull/332 we added the Repo's name (via a label) to ownership errors.

We didn't add it to timeout errors yet because it's more complex, but José expressed interest in doing it later - see comment thread.

For this issue, I just want to document the difference with a simple reproduction:

defmodule DbConnectionErrorTest do
  use ExUnit.Case
  alias MyApp.Repo

  test "Unhandled case: trigger timeout error" do
    Ecto.Adapters.SQL.Sandbox.checkout(Repo, ownership_timeout: 100)

    Repo.transact(fn ->
      Repo.query!("CREATE TEMP TABLE dogs (name text) ON COMMIT DROP")

      # Hold the connection longer than the ownership_timeout above.
      Process.sleep(200)

      # Raises an exception that does not include the repo name
      Repo.insert_all("dogs", [%{name: "Barkley"}])
      {:ok, :ok}
    end)
  end

  test "Handled case: trigger ownership error" do
    # Raises an exception that includes the Repo name (as of PR 332)
    spawn(fn ->
      Process.sleep(1_000)
      Repo.query!("SELECT 1")
    end)

    on_exit(fn -> Process.sleep(1500) end)
  end
end