`apply_action` if you want errors on your elixir form with embeds

mumble_component.ex

  def handle_event("name-change", params, socket) do
    # 1. fetch the previously assigned changeset
    export_changeset = socket.assigns.export_changeset
    # 2. apply the changes
    export_changeset = Export.changeset(export_changeset, params["export"])
    # 3. the parent changeset is invalid, and there are errors in the child (embedded) changesets,
    #    but the parent changeset does not have errors
    # 4. `apply_action` sets the action, and possibly other changes on the changeset
    case Ecto.Changeset.apply_action(export_changeset, :insert) do
      {:ok, data} ->
        {:noreply, assign(socket, export_changeset: export_changeset)}

      {:error, export_changeset} ->
        # 5. now `inputs_for` has a top level errors value,
        #    and the error tag gets displayed
        {:noreply, assign(socket, export_changeset: export_changeset)}
    end
  end