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 is kind of a big TODO. It feels like this unittest is doing too much, making it hard to get at the test we really want. Why not something simpler which lays down several writes with 5ns timestamp spacings and then experiments with txns using clocks set to different times and offsets?

Also, how about my suggestion in the original PR review:

- Create 3 clocks, each with max drift (offset) set to 100ms
- Set clock 1 to time=t, clock 2 to time=t+50ms, clock 3 to time=t+100ms
- Write three values at current time according to each of the three clocks
- Start three txns, each using one of the three clocks. In each txn, read the three values and ensure that all txns read the correct values.

The nice thing about this test is it provides a more concrete simulation of the range of readers in a cluster, separated by max clock offset and verifies they nevertheless all read the latest value.