Field deletion shall be recursive, and include steps whenver is the case.

Fix model to properly delete first childs and then itself.
Fix handler to reflect those changes, and act also on the step associated
whenever is the case.
Warning: still have to make unittests on this.
Implementing handlers.base.get_model_or_404.

* Adding error ModelNotFound, with optional parameter the model (used only to
  give more informations). This model supposedly will replace all the others
  <<MODEL>>IdNotFound stuff hangin around the code.
* Adding get_model_or_404, a helper function that fetches the model. If it is
  not found, raises a 404 HTTP error. This will supposedly replace all those if
  statements with error logs hanging around the code.
* Adapting tests to reflect these changes.