[Ivette] Adds a better error message when a request is rejected by Frama-C.

Also adds a warning from the Frama-C server.

ivette/src/frama-c/server.ts

@@ -880,7 +880,10 @@ client.onKilled((id: string) => {
 client.onRejected((id: string) => {
   const p = pending.get(id);
   if (p) {
-    p.reject('rejected');
+    const err =
+     'Request rejected by Frama-C: the related plugin may not be loaded, ' +
+     'or the Frama-C version is incompatible.';
+    p.reject(err);
     _resolved(id);
     logModel.registerRequest(
       { rid: id, statut: statutLog.REJECTED }

src/plugins/server/main.ml

@@ -279,6 +279,7 @@ let process_request (server : 'a server) (request : 'a request) : unit =
     begin
       match find request with
       | None ->
+        Senv.warning ~once:true "Unknown request %s" request ;
         Senv.debug "Rejected %a" server.pretty id ;
         Queue.push (`Rejected id) server.q_out
       | Some( kind , handler ) ->