title: Python custom exception and sub-exceptions
body: <p>What is a good way of raising sub-exceptions (is that's the term)? 
  Scenario:
  I want to raise a custom ConnectivityException when http or ftp exception occurs. Is there any way to raise ConnectivityException such that exceptions are categorized properly (i.e. I should be able to tell if ConnectivityException is raised because of http ot ftp)?</p>