Initializers aren't run on gem dependency failure

If you don't have a gem you've added to config.gem, your server starts with a warning, but your config/initializers/*.rb files aren't run. It looks like your after_initialize block isn't run, either.

If your app should refuse to run on a missing gem, that's fine, but you should throw an exception. Personally, I'm fine with the current behavior, where you just get a warning. However, the current behavior makes no sense and it's a pain to troubleshoot.

I've also included a test for the initializers folder behavior, since I couldn't find an existing test anywhere.