Catch xref errors and report them along with source file information

Currently, if an xref macro target cannot be resolved, no error is emitted and the HTML contains inappropriate markup. For example, with this markup:

xref:reference:crud.adoc[CRUD]

and the target doesn't exist in the specified module, the HTML contains:

<a href="#">reference:crud.adoc</a>

This should, IMHO, always be flagged as an error, and cause Antora to exit with a non-zero exit code.