Refactor sage export forms logic.

Everything happens in __call__()
Prepare forms list in a dedicated method (_forms_dict()).
Document /comment the method a bit.
Mark a few lines as enhanceable (FIXME)
Factor the validation of forms (only one is validated) and generally, the
method.
Fix #34 : Sage exports handle errors in form validations

On form validations, if the validation fails, a 500 page is shown
because we try to use an undefined var.
Here we fix it, error forms are now displayed.