message: ProgbarLogger: Do not crash if on_begin_batch is not called.
  on_begin_batch is not called if an exception is thrown by DistributionStrategy when distributing the dataset. Before this change, the exception would be replaced by a ProgbarLogger exception.
diff: @@ -716,6 +716,7 @@ class ProgbarLogger(Callback):
  else:
  raise ValueError('Unknown `count_mode`: ' + str(count_mode))
  self.stateful_metrics = set(stateful_metrics or [])
  + self.log_values = None
  def on_train_begin(self, logs=None):
  self.verbose = self.params['verbose']
  