#150 resolveTopicElementRef() fails on failure to resolve containing document

[W. Eliot Kimber]

This code can fail:

<xsl:variable name="containingDoc"
  as="document-node()"
  select="if ($resourcePart = '') 
  then root($context) 
  else document($resourcePart, $context)
  "
/>

If the URL can't be resolved.

Needs to trap that error and report it.

[W. Eliot Kimber]

Released in 0.9.19RC12