Make sure a `load_path` actually exists before creating an importer.

In updating Middleman to support Compass v1, I've run across this "regression."

Basically, if a project is using Compass, but doesn't have a `sass_dir` that exists, Sass explodes trying to blindly read a path that doesn't exist.

Our use-case is this: we bundle Compass with Middleman, but not everyone actually uses a specific sass directory. Rather, they embed Sass in templates directly.

I think the actual regression is in Sass, which shouldn't try to use an importer that is pointing to a missing folder.

What do you think? Basically the importer should be omitted before passing to Sass, so maybe it's correct to patch it here.