return errors in json format

lib/api.rb

@@ -3,6 +3,7 @@ Dir["#{Rails.root}/lib/api/*.rb"].each {|file| require file}
 module Gitlab
   class API < Grape::API
     format :json
+    error_format :json
     helpers APIHelpers
 
     mount Users

lib/api/helpers.rb

@@ -5,7 +5,7 @@ module Gitlab
     end
 
     def authenticate!
-      error!('401 Unauthorized', 401) unless current_user
+      error!({'message' => '401 Unauthorized'}, 401) unless current_user
     end
   end
 end