Refactor codebase to better handle success and failure

Currently the failure handling is pretty flat.

If nothing is posted back from Flo2Cash then there is no nice and clean error message explaining what happened and no easy way to try again.

Let's give some user friendly responses and also give a link that users can click to give things another go. We will need to add some validation so that once Drupal is happy payment has been received it won't try again.

In order to do this most of the codebase will be refactored and a bunch of extra features will be added.