Show CalDAV / Webcal / HTTPS calendar subscription errors directly in the "New Calendar" UI

Let the user know what went wrong, using a secondary icon and tooltip directly in the URL entry field. Also provide a hint to GTK that it is meant to be a URL field.

This will make troubleshooting and debugging much easier for our users, who traditionally encountered many bugs with the web calendars subscription feature, as the app was aware of the errors but just didn't present them.

Fixes #1177 (closed)

Complements !416 (merged) and !417 (merged).

I have been unable to encounter a case where this code breaks.
This is how it behaves in various situations:

GNOME_Calendar_new_webcalendars_subscription_error_messages_handling_testing_demonstration

Could it theoretically be better? Maybe.
Is this iterative enhancement better than nothing? It sure is.