make include_errors option work (previously always excluded)

fixes http://manage.dimagi.com/default.asp?55915

NOTE: Previously including duplicates was the *default* but it didn't actually include duplicates. Now it's off by default but the when it's on it actually works. That results in the awkward behavior that most people who had it checked by mistake will now see duplicates in their exports. Unsure what the correct way to proceed is, but one possibility is that it doesn't much matter and they can uncheck the box easily enough.