[kernel] Usable_emitter.get returns [orphan] when the emitter does not exists.

Instead of raising a [Not_found] exception.
Fixes crashes when loading a Frama-C save when an original emitter is not
available.

src/kernel_services/ast_data/annotations.ml

@@ -365,14 +365,7 @@ let code_annot_emitter ?filter stmt =
   try
     let tbl = Code_annots.find stmt in
     let filter e l acc =
-      let e =
-        try Emitter.Usable_emitter.get e
-        with Not_found ->
-          (* in some cases, e.g. when loading a state with a different set
-             of plugins loaded, the original emitter might not be available,
-             leading to discarding annotations. Let the kernel adopt them. *)
-          Emitter.orphan
-      in
+      let e = Emitter.Usable_emitter.get e in
       match filter with
       | None -> List.map (fun a -> a, e) l @ acc
       | Some f ->

src/kernel_services/plugin_entry_points/emitter.ml

@@ -155,7 +155,7 @@ module Usable_emitter = struct
         let mem_project = Datatype.never_any_project
       end)
 
-  let get e =
+  let unsafe_get e =
     let get_params map =
       Datatype.String.Map.fold
         (fun s _ acc -> Typed_parameter.get s :: acc)
@@ -167,6 +167,13 @@ module Usable_emitter = struct
       correctness_parameters = get_params e.correctness_values;
       tuning_parameters = get_params e.tuning_values }
 
+  (* In some cases, e.g. when loading a state with a different set
+     of plugins loaded, the original emitter might not be available,
+     leading to discarding annotations. Let the kernel adopt them. *)
+  let get e =
+    try unsafe_get e
+    with Not_found -> orphan
+
   let get_name e = e.u_name
   let get_unique_name e = Format.asprintf "%a" pretty e