log index-update failures

[HaberR]

Hi, thank you for this project! It is a huge help to us for interacting with our gcs helm repository.

We have a pretty large helm repository at this point which is updated often, and it seems that the helm gcs commands have degraded in performance as a result (noticing timeouts on helm gcs-push). Unfortunately, I don't have much in the way of metrics to identify what's causing this degradation. Wondering if you could add some logging around index-update failures/retries/execution times to better enable us to debug. In particular, I'd be curious about information around how long a helm gcs push takes, as well as how often we have to retry due to an index collision.