Chances are that by the time it gets to loading the package, the Error class is already loaded. Which is why the documentation states you should do this overloading in your app bootstrap.
What does a
var_dump(class_exist('Error', false));
give you in your package bootstrap? If true, then Error is already loaded and aliased.
You could try a
Package::load('mypackage'); from your app bootstrap, and see if that works?