Steps to reproduce:

1. Login as administrator on a site and access registration page
2. Select "Register with a specific hub" option
3. Enter http://www.google.com as the hub address and press the button

Result: a form is displayed to collect the sites data. Only after having completed and submitted the form, an error is returned as the URL is not one of a valid Moodle hub.

Expected result: an error should be displayed before displaying the form, i.e. the URL should be verified BEFORE displaying the form, raising an error in case of non-valid hub URL.