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.
Not even a TODO actually - the unittest is already doing that check just a few lines below - we know the spacing, how many values there are and how many of the following values bother us. It's just the TODO that's old. I've added your test case in the test call for good measure - agreed that I wouldn't want to attach any more bells and whistles to this test though.