Allow that zk NOTEMPTY multi exception is retryable by running in-series

It looks like our zk behavior changed slightly on upgrade. In tests on occasion, I see a fail because the zk delete of dir fails because dir is not empty. Or, this flakey type we just ignored in the past (its rare enough – I'm just seeing it of late because I'm running tests back-to-back on 4 different hardwares). Let me try allowing NOTEMPTY as a retryable. Lets see how it does.

Threw in some debug while at it for other issues I've seen in test.