Fix train and generate directories getting created

This addresses issue https://github.com/ibab/tensorflow-wavenet/issues/364

train.py was attempting to write to the 'train' directory and throwing a NotFoundError here: https://github.com/ibab/tensorflow-wavenet/blob/master/train.py#L274 which could happen before the makedirs() line in the save function was called.

generate.py did not have a call to makedirs() and so it was possible to get a NotFoundError here: https://github.com/ibab/tensorflow-wavenet/blob/master/generate.py#L263