add txn uncertainty test & fix a bug

Added missing Txn Uncertainty test and along the way realized that forgetting to add ReadWithinUncertaintyIntervalError to GoError() is not a great thing to do as the error simply evaporates.
Yep, looks like the RetryReset in util/retry.go didn't actually retry immediately but kept a small backoff (via jitter) which became fairly large in my case since I set the Backoff to 10sec to make sure they never happen.