ProxyBackend form notification, if form is not filled at all

[jykae]

Found by @marla-singer in PR #1806

Steps:

1. Go to any API and open Proxy tab
2. Click on Delete button, if there are any information
3. Don't fill a form and click on Save&Publish button

Found result: Submit hook isn't end, button is disable, no one notification about required fields
Expected result: Notify users about required fields and button must be available