Make it a fatal error if 'dir' is not set for gitaly-ruby or gitlab-shell

We have 'gitaly-ruby' and 'gitlab-shell' sections in our config.toml file, both with a 'dir' attribute. For the sake of backwards compatibility these fields may still be empty. Now that this code has been integrated upstream we should make it a fatal error (abort start-up) if either of these does not point to a directory.