should reject an scxml document containing assign without @location

[mattoshry]

Per [1], assign/@location is required.

SCION should reject the following:

<scxml xmlns="http://www.w3.org/2005/07/scxml"
  version="1.0">

<state id="uber">

  <onentry>
    <!-- missing location -->
    <assign expr="'foo'"/>
  </onentry>

  <transition event="*" target="fail">
    <log expr="'unhandled event ' + JSON.stringify(_event)"/>
  </transition>

</state>

<final id="fail"/>

</scxml>

[1] https://www.w3.org/TR/scxml/#assign