immutable_drop: use custom error for bad set

lib/jekyll.rb

@@ -32,6 +32,7 @@ require 'colorator'
 SafeYAML::OPTIONS[:suppress_warnings] = true
 
 module Jekyll
+  StandardError = Class.new(::StandardError)
 
   # internal requires
   autoload :Cleaner,             'jekyll/cleaner'

lib/jekyll/drops/immutable_drop.rb

@@ -3,6 +3,7 @@
 module Jekyll
   module Drops
     class ImmutableDrop < Liquid::Drop
+      IllegalDropModification = Class.new(Jekyll::StandardError)
 
       def initialize(obj)
         @obj = obj
@@ -18,7 +19,7 @@ module Jekyll
 
       def []=(key, val)
         if respond_to? key
-          raise ArgumentError.new("Key #{key} cannot be set in the drop.")
+          raise IllegalDropModification.new("Key #{key} cannot be set in the drop.")
         else
           fallback_data[key] = val
         end