Crash on broken internal query without index

Based on https://github.com/gjtorikian/html-proofer/issues/41#issuecomment-52594922 by @nschonni

I will try to fix it here.
> And maybe I would suggest renaming the option to: directory_index_file.

I borrowed the name from Apache http://httpd.apache.org/docs/current/mod/mod_dir.html#directoryindex