bridge_softmix/sfu_topologies_on_join: Ignore topology change failures

When a channel joins a bridge, we do topology change requests on all
existing channels to add the new participant to them.  However the
announcer channel will return an error because it doesn't support
topology in the first place.  Unfortunately, there doesn't seem to be a
reliable way to tell if the error is expected or not so the error is
ignored for all channels.  If the request fails on a "real" channel,
that channel just won't get the new participant's video.

Change-Id: Ic95db4683f27d224c1869fe887795d6b9fdea4f0