Quicker load and no debug print statement

Thanks for a very useful project!! :)

I've removed an annoying print statement in `auto_discover()` regarding registered models. I don't find it useful since it's very easy to debug whether a model is registered or not, and I think the app/developer can live without it.

The catch-reraise pattern on non-`ImportError` has also been reduced to avoid first raising an ImportError and then detecting if there is a sub-module (which there obviously isn't when it's an ImportError).