I fixed the bug in two `DeleteView` views. The `SuccessMessageMixin`
doesn't work with `DeleteView` views because it's hooked to the
`form_valid` method which doesn't exist in this particular view.

I took the opportunity to rewrite the other success or error messages in
the other views in order to have something a bit clearer and easier to
read.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>