Proto: wrong/unclear doc strings in error declarations

There's an inconsistency in contract_storage.ml regarding the error Empty_transaction. It is meant to be Temporary but then it is declared as Branch:

  | (* `Temporary *)
      Empty_transaction of Contract_repr.t (* `Temporary *)

...

  register_error_kind
    `Branch
    ~id:"contract.empty_transaction"

The same issue might occur elsewhere.

Also, probably due to the introduction of ocamlformat formatting, it is often unclear to which errors do the doc strings apply.