title: Which exception should be raised when a required environment variable is missing?
body: <p>The title pretty much sums it up already.</p>
  
  <p>I have a piece of code that calls <code>os.getenv</code> to get both a URL as well as a token in order to connect to a service. The code lives in a module and will only be imported from there, i.e. it's not a script. </p>
  
  <p>It's not a huge issue at all, since I really only need to crash and display the message saying that there are unset values, but it got me thinking about which of Python's built-in exceptions would be the best fit.</p>
  
  <p>I found the <code>EnvironmentError</code>, but that seems to function as base class from which <code>IOError</code> and other OS related exceptions inherit.</p>
  
  <p>Would it be as simple as a <code>ValueError</code>, as it's really just a value that's missing?</p>
  
  <p>Thanks!</p>