Only ensure against yield so that pool should be available

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9192#note_23293693

lib/gitlab/database.rb

@@ -72,11 +72,12 @@ module Gitlab
     def self.with_connection_pool(pool_size)
       pool = create_connection_pool(pool_size)
 
+      begin
         yield(pool)
-
       ensure
         pool.disconnect!
       end
+    end
 
     def self.create_connection_pool(pool_size)
       # See activerecord-4.2.7.1/lib/active_record/connection_adapters/connection_specification.rb