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.
This expression looks rickety in terms of operator precedence. Is it supposed to be:

(int(maxOffset) / 5) * (1 - (i+1)/concurrency)

or

int(maxOffset) / (5 * (1 - (i+1)/concurrency))

Add some parentheses one way or another.