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.
@spencerkimball The next line takes about a second, and it's not the parallelism. It looks like it's the txn restart (which doesn't seem to be backing off, but going at it immediately, also there aren't more of them than there should be - just a single one).