make capability validation message for URL type fields consistent and less confusing

Base URL capability has a validation message for the URL field "must be a valid URL" - rest of the site has 'This field should be a URL in the format "http://www.example.com"'.
I looked to make it consistent but couldn't find it in github so I am ticketing instead.
I made this a bug rather than an improvement because I think the level of confusion is potentially high if you enter "www.example.com" and it asks for a "valid URL".
I did not check older NX3 or NX2 at this time.