added an exception message when trying to use a non Propel class with sfPropelData::load() method
added an exception message when trying to use a non Propel class with sfPropelData::load() method

git-svn-id: 2c1cedb23b8580241ed92ba4ad7c1ac34af28f97@5839 ee427ae8-e902-0410-961c-c3ed070cd9f9

Symfony-Plugins/sfPropel14Plugin