REGISTRAR, GUI: do not allow submit form if req fed attrs are missing

REGISTRAR:

- Throw custom exception if user is requesting application form and any of form items is:
   - required &&
   - not editable by user (from_federation_show/hidden) &&
   - it's pre-filled value provided by IDP is missing.
- Exception is thrown after all items are processed to show all missing attributes.
- Exception is thrown only when user is using federated authz.

GUI:

- Handle custom exception for case mentioned above and display custom error text
  on application form page. Therefore no form is displayed and can't be submitted.