exclude missing modules when ignoreMissing is set

This makes so that `require` calls to missing modules now throw instead of returning `{}` when the `--ignore-missing` flag is set, as reported on https://github.com/substack/node-browserify/issues/733.

I believe this behavior better matches the node.js semantics.

I noticed though that through the API there's a distinction between `ignore` and `exclude`. Perhaps it's a better idea to create a `--exclude-missing` flag to make this distinction fully orthogonal across the API?