Use `raise` instead of logging & aborting

lib/jekyll/filters.rb

@@ -207,9 +207,8 @@ module Jekyll
         when nils == "last"
           order = + 1
         else
-          Jekyll.logger.error "Invalid nils order:",
+          raise ArgumentError.new("Invalid nils order: " +
-            "'#{nils}' is not a valid nils order. It must be 'first' or 'last'."
+            "'#{nils}' is not a valid nils order. It must be 'first' or 'last'.")
-          exit(1)
         end
 
         input.sort { |apple, orange|