Handles wrapped excon errors as heroku api errors

Yesterday, heroku api was down. We have lost a lot of our background jobs. For more details see my issue https://github.com/JustinLove/autoscaler/issues/30.
I saw, that you rescue from excon exceptions already, but it does not work for `Heroku::API::Errors` classes, which wrap excon errors. So, my code rescues from `Heroku::API::Errors::Error` class, which is the base class for all heroku api exceptions.