* Set default `Error` status code to 404
This is an appropriate default behavior because:
1. When the server encounters an error, the `err` property is set.
2. When the client-side application crashes, the `err` property is set.
This means the "only" way to render the `/_error` page without an error
is when a page is not found (special condition).
Fixes#6243Closes#5437
* Add new integration test for client side 404
* single quotes
* Remove unused variable
* Standard needs to go away
* Whoops
* Check for null status code in res and err
* Only check response for valid statusCode
