title: What is the correct way to stop a checkout from an event observer in Magento?
body: <p>I am verifying shipping quotes during the event checkout_controller_onepage_save_shipping_method and if the verification fails i want to send the user back to shipping method selection but I would also like to display a message saying why it failed. Does Magento have a way of doing this built in?</p>
  
  <p>I am already verifying the data I just lack the redirect to shipping methods and way to display a message.</p>