Work around Jekyll error-handling bug which led to silent doc build failures

If a custom Jekyll template tag throws Ruby's equivalent of a "file not found" exception, then Jekyll will stop the doc building process but will exit with a successful status, causing our doc publishing jobs to silently fail.

This is caused by https://github.com/jekyll/jekyll/issues/5104, a case of bad error-handling logic in Jekyll. We should work around this in Spark to avoid more silent documentation build breaks.

(See SPARK-16553 for an example of a build failure which was ignored due to this Jekyll bug).