title: how to check if shopify session has expired?
body: <p>say I have this call in my code:
  <code><pre>
  products = shopify.Product.find()
  </pre></code>
  which works if I am just logged in, but after some time it starts failing with an error saying <code>AttributeError: 'NoneType' object has no attribute 'find'</code>. I think this happens because the session expires and my code does not check that.</p>
  
  <p>is there a way to check if shopify session has expired? I store the store URL and the authentication token in my webapp2 session thus:
  <code><pre>
  self.session['shopify'] = {
      'shop_url': shopify_session.url,
      'access_token': shopify_session.token
      }
  </pre></code>
  are there any other properties that I need to store in my web session that I can use to check if the shopify session has expired?</p>