/wp-json/wp/v2/menus incorrectly emits 403 for invalid `post` param

[jkmassel]

This endpoint expects the post param to be the Post ID for a nav_menu_item, but if the Post ID for a post/page/etc is provided the following response is returned:

{
  "code": "rest_forbidden_context",
  "message": "Sorry, you are not allowed to view terms for this post.",
  "data": {
    "status": 403
  }
}

It should probably look more like:

{
  "code": "rest_post_invalid_type",
  "message": "Invalid post type.",
  "data": {
    "status": 400
  }
}

The wp/v2/menus API will return a 403 if the post parameter refers to a post object that isn’t a nav_menu_item. This isn’t a permissions issue – it’s just an invalid request, and the response should reflect that.

Trac ticket: https://core.trac.wordpress.org/ticket/64045