Scan asciidoc output for errors
Scan asciidoc output for errors

Summary:
We should fail asciidoc targets if they
invoke filters that fail. This will make
building the schema document fail if one
of the examples doesn't verify.

Reviewers: shahms, schroederc

Reviewed By: schroederc

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D299

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