Check to see if relevance and constraint values refer to existing nodes

**Issue by [mitchellsundt](https://github.com/mitchellsundt)**
_Thursday Jul 09, 2015 at 19:23 GMT_
_Originally opened as https://github.com/opendatakit/opendatakit/issues/835 (0 comment(s))_

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Originally reported on Google Code with ID 834

```
"in my latest version of ODK validate (V. 1.2.3 for ODK Collect 1.2), variables inside
constraint= and relevant= and calculate= clauses are not checked for existence. Is
there a version of Validate coming out soon that does this work? It is rather time
consuming to go through every relevant= clause and check that all the variables have
been spelt correctly."
-- tclucretius@gmail.com
```

Reported by `yanokwa@nafundi.com` on 2013-05-27 01:20:43