message: Make --output_prefix required, no default
diff: @@ -46,8 +46,9 @@ flags.DEFINE_float('training_fresh', 0.05,
  flags.DEFINE_integer('batch_size', 1024,
  'How many TFRecords to pull through tf.Session at a time')
  -flags.DEFINE_string('output_prefix', 'gs://dtj-minigo-us-central1/tryit_',
  +flags.DEFINE_string('output_prefix', None,
  'Name of output file to receive TFRecords')
  +flags.mark_flag_as_required('output_prefix')
  flags.DEFINE_integer('concurrency', 4,
  'Number of parallel subprocesses')
  