If error is thrown when loading AsciiDoc document and --stacktrace option is used, CLI shows undefined in console

If an error is thrown somewhere inside of Asciidoctor or a user extension when the AsciiDoc document is being loaded, and the CLI is invoked with the --stacktrace option, the user will see undefined in the console. This happens because the stack property on the error object is not populated in this case. The CLI should detect this scenario and look for the information elsewhere with a sane fallback.