carddav: consider falling back to input domain in Discover

[emersion]

trafficstars

If there's no SRV record, maybe we should default to the input domain, as suggested in the RFC?

   *  If an SRV record is not found, the client will need to prompt
      the user to enter the FQDN and port number information
      directly or use some other heuristic, for example, using the
      extracted "domain" as the FQDN and default HTTPS or HTTP port
      numbers.

Or at least return a fixed error value which can be handled by the user.