Raises exception if .erb extension is forgotten

If now we use erb tags but don't add erb extension to the file name,
error message is quite obscure:

  Invalid CSS after "...und-image: url(": expected ")", was "<%=
  asset_path ..."

This raises a more meaningful message like:

  You are using erb in your file: #{filename} but have not added a .erb
  extension.

Related notes: https://gist.github.com/schneems/5249586