Butler fails silently on YAML syntax errors

[froboy]

When Butler attempts to compile faulty YAML files it fails silently. It should notify the user of the error, preferably what line of which file is failing.

I've tried implementing this with something like https://github.com/dtothefp/gulp-yaml-validate, but I wasn't able to integrate it into Butler's Gulpfile.js.

Optimally, I see this running before or after the CSS linter, scanning styleguide/source/styleguide/*.html for YAML parsing errors, and then outputting the errors to the terminal.