chore(nat, upnp): Make failure logging less reptitive (ref #9324) (#9785)

Currently we log on every single one of 10 retries deep in the upnp
stack. However we also return the failure as an error, which is bubbled
up a while until it's logged at debug level. Switch that around, such
that the repeat logging happens at debug level but the top-level happens
at info. There's some chance that this will newly log errors from
nat-pmp that were previously hidden in debug level - I hope those are
useful and not too numerous.

Also potentially this can even close #9324, my (very limited)
understanding of the reports/discussion there is that there's likely no
problem with syncthing beyond the excessive logging, it's some weird
router behaviour.

lib/upnp/igd_service.go

@@ -206,7 +206,7 @@ func (s *IGDService) AddPortMapping(ctx context.Context, protocol nat.Protocol,
 		}
 
 		err = fmt.Errorf("UPnP Error: %s (%d)", envelope.ErrorDescription, envelope.ErrorCode)
-		l.Infof("Couldn't add port mapping for %s (external port %d -> internal port %d/%s): %s", s.LocalIPv4, externalPort, internalPort, protocol, err)
+		l.Debugf("Couldn't add port mapping for %s (external port %d -> internal port %d/%s): %s", s.LocalIPv4, externalPort, internalPort, protocol, err)
 	}
 
 	return externalPort, err