Proto: remove usage of the Not_found exception 🐔

Context

The Not_found exception was used by the protocol, for example in https://gitlab.com/tezos/tezos/-/blob/master/src/proto_alpha/lib_protocol/contract_services.ml However, this exception does not appear in the environment (after inspection by @raphael-proust, it seems this exception is hard-coded into the OCaml environment). Moreover, exceptions cannot be translated by the coq-of-ocaml tool as, unlike monads, these side-effects are not traced by the OCaml compiler.

This MR remove the use of the Not_found exception in the protocol.

Manually testing the MR

I do not know how to manually test these changes.