better error msgs

Author: jchavarri

ppx/browser/ppx_deriving_json_runtime.ml

@@ -76,8 +76,8 @@ module Of_json = struct
       let v = (Obj.magic json : string) in
       match Int64.of_string_opt v with
       | Some v -> v
-      | None -> of_json_error "expected an int64"
-    else of_json_error "expected a string"
+      | None -> of_json_error "expected int64 as string"
+    else of_json_error "expected int64 as string"
 
   let float_of_json (json : t) : float =
     if Js.typeof json = "number" then (Obj.magic json : float)

ppx/native/ppx_deriving_json_runtime.ml

@@ -73,8 +73,8 @@ module Of_json = struct
     | `String i as json -> (
         match Int64.of_string_opt i with
         | Some v -> v
-        | None -> of_json_error_type_mismatch json "string")
-    | json -> of_json_error_type_mismatch json "string"
+        | None -> of_json_error_type_mismatch json "int64 as string")
+    | json -> of_json_error_type_mismatch json "int64 as string"
 
   let float_of_json = function
     | `Float f -> f