Fix race condition between http error notification and server-side validation error notification

[slax57]

Problem

When validating server side, you can’t provide a global error message and react-admin displays a default non customizable one.

In some cases (depending on the React version) users could use the error's message field to render a custom message in the notification, but due to a race condition between the 2 notifications, it would not always work.

Solution

In useCreateController and useEditController, check if there are validation errors and if we are in pessimistic mode, and don't notify in this case (as it will be done by useNotifyIsFormInvalid).

In useNotifyIsFormInvalid notify with the root.serverError message if available (otherwise fall back to 'ra.message.invalid_form' like currently).

[djhi]

💪