Fixed #4371 -- Improved error checking when loading fixtures. Code now catches...

Fixed #4371 -- Improved error checking when loading fixtures. Code now catches explicitly named fixture formats that are not supported (e.g, YAML fixtures if you don't have PyYAML installed), and fixtures that are empty (which can happen due to XML tag errors). Thanks to John Shaffer for the suggestion, and Keith Bussell for his work on the fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7595 bcc190cf-cafb-0310-a4f2-bffc1f526a37