Specialized error for `hess_coord` and `hess_structure` with `FeasibilityResidual`

[tmigot]

I was trying to find a feasible point of an NLPModel by transforming it into a FeasibilityResidual and then using ipopt, which returns an error because hess_coord and hess_structure are not implemented for FeasibilityResidual.

I know this has been discussed for LLSModel already in #273. If I understand correctly, the fix is to call ipopt(FeasbilityFormNLS(FeasibilityResidual(nlp))).

If this fix is "well-accepted", we should probably return a specialized error for hess_coord and hess_structure that document the use of FeasbilityFormNLS instead of the NotImplementedError. What do you think?

[dpo]

FeasbilityFormNLS(FeasibilityResidual(nlp))

One of my favorite constructions! Yes I agree we should document that better!