Fixes #1812 -- Added model validity checks to ensure that models.py exists,...

Fixes #1812 -- Added model validity checks to ensure that models.py exists, and has been successfully imported for all INSTALLED_APPS. Previous behaviour was to silently ignore empty/problem models, which resulted in the display of an admin page that doesn't display a supposedly installed model. Thanks to Ian Holsman for the original report.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3201 bcc190cf-cafb-0310-a4f2-bffc1f526a37