Dev - Bail if YAML/Markdown Parsing Fails

[bebraw]

Otherwise pages might end up being skipped from the build.

[skipjack]

Is there a way to do a dry run or to fail out of the build process without antwar edits? It seems we could just throw some errors within the markdown utility or antwar.config.js?

[skipjack]

Noted in #1380 -- let's track there.