Update StochasticGradient.lua

Added default `true` `self.verbose` which will keep outputting the "current error" at every iteration by default (but now it can be disabled)
Computed `currentError` before `hookIteration` so that it can be passed to the function (for printing error rate vs. iteration, for example)
Printed final "training error" (just in case we have disabled the `verbose` option)