make org-ql more robust to badly formed habits

[codygman]

Given an org file:

* TODO meditation
  SCHEDULED: <2015-01-30 Fri 08:00 .+1d>
  :PROPERTIES:
  :STYLE:        habit
  :END:
* TODO jog
  SCHEDULED: <2015-01-28 Wed 19:30-19:45 .+1d>

This org-ql query:

(org-ql FILE (scheduled <= today))

gives error:

if: Unknown date-element type: active-range

I think it would be better if this error didn't break the org-ql function.

After adding the habit property things work fine, but it would be more robust to return results if possible.