New common template error.tpl

Hi!

I developed a payment gateway time ago (now I´ve just released a new version), and one thing I do understand, is why there isn´t a template of error, only a template of success. I know a template is not a core file, but all the themes are based on the files of the default theme. It´s not a difficult thing, in fact I was thinking to made an error template and make a pull request to the opencart github, but I find it odd no one has asked himseld: well, if the payment goes wll, you redirect the costumer to success. if the payment is denied/failed/..., what? Most payment gateways redirect to success page, but it has no sense. If the payment is denied, it´s not a success order...

I´ve made a pull request of this feature. If anoyone wants to have a look: https://github.com/opencart/opencart/pull/216