Code review comment for ~nacc/git-ubuntu:importer-cleanup-tmpdir-always

I can confirm based on a cursory overview of the code that MockError having a superclass of RuntimeError will result in any `except RuntimeError` captures picking up MockError in that class.

And per Robie's comment, MockError should probably inherit from Exception or BaseException if you want it to be handled as its own class of exceptions outside that of RuntimeError.