Add EngineControl::stop_engine method to show errors when stopping engine

Add EngineControl::stop_engine method to show errors when stopping engine

This replaces using ARDOUR_UI::disconnect_from_session which is only used by the
EngineControl class. ARDOUR_UI::disconnect_from_session also disconnects from
the AudioEngine::Halted signal which seems unnecessary as Halted is not emitted
when stopping the engine and calling update_sample_rate() which is already
handled when the AudioEngine::Stopped signal is emitted.