Give better feedback if ini files are busted

The goal here is to protect against a common problem: the .ini files in the `cache` folder get messed up for whatever reason during an upgrade and it completely baffles the user with an error like "Fatal error: Class 'Gdn' not found".

Examples: http://vanillaforums.org/search?adv=&search=%22Fatal+error%3A+Class+%27Gdn%27+not+found%22&title=&author=&cat=all&tags=&discussion_d=1&discussion_question=1&comment_c=1&comment_answer=1&page_p=1&within=1+day&date=

This is the most straightforward way I could see of making this an easy issue to sidestep for our open source users, but I'm not sure it's the best solution. Suggestions welcome.