'throw' will raise a synchronous exception which bypasses all the error
handling based on defer/promises, triggering an "Ooops!" error window of
cockpit, and leaving an in-progress register dialog.

Instead, use the standard defer/promise error handling done in other
parts of the registerSystem() function already, so the error message
will be properly shown as inline error in the register dialog.

Reword the text of some untranslated error messages to be more
"elegant", for example with the proper spelling of "IPv6" & "URL",
and by removing the ellipsis (which fit "in progress" messages).

Also, use the gettext function to properly translate them.

This fixes the cockpit parts of
https://bugzilla.redhat.com/show_bug.cgi?id=2076914