fail gracefully if "sass" gem cannot be loaded (#6573)
fail gracefully if "sass" gem cannot be loaded (#6573)

Merge pull request 6573
getandpost/jekyll.github.io,jekyll/jekyll,gencer/jekyll,x-way/jekyll,tillgrallert/jekyll,Strangehill/jekyll,ryanshaw/jekyll,fulldecent/jekyll,x-way/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,x-way/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,gencer/jekyll,getandpost/jekyll.github.io,yhironaka/yhironaka.github.io,chrisfinazzo/jekyll,Strangehill/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,tillgrallert/jekyll,x-way/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,Strangehill/jekyll,tillgrallert/jekyll,fulldecent/jekyll,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,gencer/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,gencer/jekyll,jekyll/jekyll,Strangehill/jekyll,fulldecent/jekyll,jaybe-jekyll/jekyll,gencer/jekyll,jekyll/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll