Adjust error range for RelaxNG "not allowed yet" when there is choice

[datho7561]

Given the following .rng:

<grammar xmlns="http://relaxng.org/ns/structure/1.0"
  datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
  <start>
    <ref name="rootElt" />
  </start>
  <define name="rootElt">
    <element name="root">
      <choice>
        <element name="elementA">
          <text />
        </element>
        <element name="elementB">
          <text />
        </element>
      </choice>
      <element name="element2">
        <text />
      </element>
    </element>
  </define>
</grammar>

and the following XML:

<?xml-model href="notAllowedYet.rng"?>
<root>
  <element2></element2>
</root>

The error code is listed as to_implement, and the error range doesn't select the start tag of element2.

[datho7561]

@JessicaJHee if you are interested, I think this could be a good issue to fix for this sprint. Most of the other issues I found that I think should be fixed for 1.0 were trivial to fix (mostly NPEs).