errors: make exit codes class variables instead Kyle pointed out to me that we can simply make the exit codes class variables. Python provides some magic for making them accessible as instance variables. This also makes it easier to let subclasses of existing errors override the exit codes by letting them simply define their own values as class variables. That means that there's no need to pass them into the superclass's constructor arguments, so the superclass doesn't need to expose the them as arguments. (Making a subclass set a different exit code for a subclass of `StorageError` was actually the goal with my recent series.) Differential Revision: https://phab.mercurial-scm.org/D10758