Add Repo name to timeout errors

[nathanl]

In #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