title: Raise an error if method's parameters do not meet certain criteria
body: <p>Is it possible to prevent an object from being initialized and raise an error if some parameters passed to the initialize method don't match certain conditions?</p>
  
  <p>For example, if I define a class <code>EvenInt</code> that has one attribute <code>x</code> that must be an even integer, is it possible to prevent an object of this class from being initialized, for instance, if I pass <code>3</code> as a parameter?</p>
  
  <p>If so, could you quickly explain how to do so, particularly how to raise errors?</p>