API: extracted helper method to provide 400 bad request error with description

Extracted a method for 400 error (Bad request) and adjusted code accordingly. The name of
the missing attribute is used to show which one was missing from the request. It is used to
give an appropriate message in the json response.

lib/api/milestones.rb

@@ -13,7 +13,7 @@ module Gitlab
         #
         def handle_milestone_errors(milestone_errors)
           if milestone_errors[:title].any?
-            error!(milestone_errors[:title], 400)
+            bad_request!(:title)
           end
         end
       end