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

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