When attempting an invalid transition on a dossier, provide a meaningful
error message (instead of letting an `AASM::InvalidTransition` exception
propagate).
This handles not only the case where the same state is applied twice
(which was already handled manually), but all cases where the transition
is invalid.
It was slightly broken in two ways:
- it doesn’t make other instructeurs unfollow the dossier
- if the dossier is unarchived, instructeurs don’t refollow automatically.
