Allow 503s and other non-customized error messages to be handled by Workhorse

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21870#note_106212058:

Now that this MR is merged, render_503 will be returning the 503 page with the override header set, so Workhorse won't be serving this file, or any error files for that matter.

I think we might want to check the options parameter for layout or template before setting this header.

/cc: @davidpiegza