immutable_drop/errors: consolidate errors & fix syntax for raising

Addresses @envygeeks's comments:
bff1726a5a

lib/jekyll.rb

@@ -33,8 +33,6 @@ require 'colorator'
 SafeYAML::OPTIONS[:suppress_warnings] = true
 
 module Jekyll
-  StandardError = Class.new(::StandardError)
-
   # internal requires
   autoload :Cleaner,             'jekyll/cleaner'
   autoload :Collection,          'jekyll/collection'

lib/jekyll/drops/immutable_drop.rb

@@ -3,8 +3,6 @@
 module Jekyll
   module Drops
     class ImmutableDrop < Liquid::Drop
-      IllegalDropModification = Class.new(Jekyll::StandardError)
-
       def initialize(obj)
         @obj = obj
       end
@@ -19,7 +17,7 @@ module Jekyll
 
       def []=(key, val)
         if respond_to? key
-          raise IllegalDropModification.new("Key #{key} cannot be set in the drop.")
+          raise DropMutationException, "Key #{key} cannot be set in the drop."
         else
           fallback_data[key] = val
         end

lib/jekyll/errors.rb

@@ -1,9 +1,8 @@
 module Jekyll
   module Errors
-    class FatalException < RuntimeError
-    end
+    FatalException = Class.new(::RuntimeError)
 
-    class MissingDependencyException < FatalException
-    end
+    MissingDependencyException = Class.new(FatalException)
+    DropMutationException      = Class.new(FatalException)
   end
 end