Fixed #6450 -- Improved the checking of errors when creating the directories...

Fixed #6450 -- Improved the checking of errors when creating the directories for saved files. Thanks to henry@precheur.org for the report and patch, and vung for the excellent test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8007 bcc190cf-cafb-0310-a4f2-bffc1f526a37