Status checks API does not return duplicate value errors on update

When creating a new status check, errors are returned by the API if a user attempts to use the same name or url as another status check. These errors are then shown in the UI.

However, if you try updating an existing status check with the same name and/or url as another status check, we get a 422 Unprocessable Entity response but with no error messages, so the UI can't show any errors to the user.

We should make sure that the duplication errors are returned by the API when updating status checks just like creating.