Error_monad: improve error message for unrecognized errors

src/lib_error_monad/error_monad.ml

@@ -413,6 +413,8 @@ module Make() = struct
           (Format.pp_print_list pp)
           (List.rev errors)
 
+
+  (** Catch all error when 'serializing' an error. *)
   type error += Unclassified of string
 
   let () =
@@ -439,6 +441,25 @@ module Make() = struct
     error_kinds :=
       Error_kind { id ; from_error ; category ; encoding_case ; pp } :: !error_kinds
 
+  (** Catch all error when 'deserializing' an error. *)
+  type error += Unregistred_error of Data_encoding.json
+
+  let () =
+    let id = "" in
+    let category = `Temporary in
+    let to_error msg = Unregistred_error msg in
+    let from_error = function
+      | Unregistred_error json -> Some json
+      | _ -> None in
+    let encoding_case =
+      let open Data_encoding in
+      case Json_only json from_error to_error in
+    let pp ppf json =
+      Format.fprintf ppf "@[<v 2>Unregistred error:@ %a@]"
+        Data_encoding.Json.pp json in
+    error_kinds :=
+      Error_kind { id ; from_error ; category ; encoding_case ; pp } :: !error_kinds
+
   type error += Assert_error of string * string
 
   let () =