Admin UI: when wrong iOS certificate passphrase is provided, no meaningful message is shown to user

* generic client-side  handling for 404 Bad Request that contains JSON object as a response body
* additional meaningful messages provided when validation fails for various Variant properties
* make choice between iOS production/development required in UI


test by providing non-sense values such as wrong certificate, wrong certificate passphrase or too long input to e.g. Windows WPNS's Package SID