Fixed #7727 -- Improved the checks for import failure when using PIL. Under...

Fixed #7727 -- Improved the checks for import failure when using PIL. Under PyPy, you can import the PIL module, but when you try to use it, the underlying _imaging module will not be available. Thanks to Maciej Fijalkowski (fijal) for the report and suggested fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8016 bcc190cf-cafb-0310-a4f2-bffc1f526a37