Catch Asciidoctor.js errors.

If an include:: target cannot be found/resolved, Asciidoctor.js emits an error, such as:

asciidoctor: ERROR: crud.adoc: line 163: include target not found: ROOT:partial$examples/crud/iteration_3.adoc

Currently, Antora simply continues operating as if there was no error and exits with an exit code of 0, confounding build systems that depend on correct status codes.

Antora should catch such errors.