remove unused error class (#6511)
remove unused error class (#6511)

Merge pull request 6511
Strangehill/jekyll,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,tillgrallert/jekyll,x-way/jekyll,fulldecent/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,ryanshaw/jekyll,yhironaka/yhironaka.github.io,jekyll/jekyll,fulldecent/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,jaybe-jekyll/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,ryanshaw/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,Strangehill/jekyll,tillgrallert/jekyll,getandpost/jekyll.github.io,tillgrallert/jekyll,fulldecent/jekyll,jekyll/jekyll,x-way/jekyll,chrisfinazzo/jekyll,x-way/jekyll,x-way/jekyll,Strangehill/jekyll