Out of curiosity, when does this happen? Is there a situation where this is likely to happen, or are we just trying to be more defensive against an obscure scenario where all we can do is shrug and move on?

It's mostly defensive here; there's a lot of code in libnetwork that either ignores failures (which could be "failed to ADD the interface to the zone"), or "if the machine is not in state X" (e.g. firewalld not running), then we skip; here's how this is currently used;

	if firewalldRunning {
		if enable {
			if err := AddInterfaceFirewalld(bridgeName); err != nil {
				return err
			}
		} else {
			if err := DelInterfaceFirewalld(bridgeName); err != nil {
				return err
			}
		}
	}

But firewalldRunning is something that can be updated on various locations, which means that the state of the world may actually change.

For a cleanup operation, I'd consider "removing what isn't there" to be something we can safely ignore.