Catch backdrop exceptions and display in admin UI

 * Change approach of using response_add and response_change to
 changelist_view which covers all scenarios
 * New approach is to delete the success message from the django
 messages queue rather than intercepting it. Because django 1.6 does
 not allow intercpeting the delete success message. django 1.7 does
 support this however if we upgrade. http://stackoverflow.com/a/18289950
 * Override log_deletion and delete_model as per existing add and
 change
 * Factor out backdrop exception handler to support both save_model and
 delete_model

https://www.pivotaltracker.com/story/show/71300964

[Fixes #71300964]
Merge pull request #138 from alphagov/handle-delete-errors-in-ui

Catch backdrop exceptions and display in admin UI