Add a more informative error message when runtests fails to import from test suite directory

[anonymous]

When trying to run the test suite after updating my repo, I was getting strange ImportErrors. It turned out there were orphaned empty directories in the tests directory. A small change that prints a hint when that happens would help avoid confusion for new and less regular contributors.

A pull request for this is available here:
​https://github.com/django/django/pull/1342

[Tim Graham]

Comments from the pull request:

Marc: "I'm fairly confident that this isn't the only reason an exception could be raised on that line. It's probably a hangover from the one-off major refactor of how the test suite is structured which shouldn't impact too many people."

me: "I agree with Marc. You can use a git post-checkout hook to automatically delete empty directories. Added the tip here: https://code.djangoproject.com/wiki/CommitterTips"