add back message key in order to have error_codes

Author: Marieke GUEYE

lib/grape/exceptions/validation.rb

@@ -9,6 +9,7 @@ class Validation < Grape::Exceptions::Base
       def initialize(args = {})
         fail 'Params are missing:' unless args.key? :params
         @params = args[:params]
+        @message_key = args[:message] if args.key?(:message) &&  args[:message].is_a?(Symbol)
         args[:message] = translate_message(args[:message]) if args.key? :message
         super
       end