particular, rather than just dumping exactly 400 of the last loss values, it
now dumps 400 + 10% of the loss buffer.  This way, the amount of the dump is
proportional to the steps without progress threshold.  This is better because
when the user sets the steps without progress to something larger it probably
means you need to look at more loss values to determine that we should stop,
so dumping more in that case ought to be better.