message: eager: Tweak error message.
  Motivated by
diff: @@ -496,9 +496,11 @@ tensorflow::Status ValidateInputTypeAndPlacement(
  expected_device->name(), " but is actually on ",
  actual_device->name(), " (operation running on ",
  op_device->name(), ")",
  - " Tensors can be copied explicitly using .gpu() or .cpu(),"
  - " or transparently copied by using tfe.enable_eager_execution("
  - "tfe.DEVICE_PLACEMENT_SILENT). Copying tensors between devices"
  + " Tensors can be copied explicitly using .gpu() or .cpu() "
  + "methods,"
  + " or transparently copied by using tf.enable_eager_execution("
  + "device_policy=tfe.DEVICE_PLACEMENT_SILENT). Copying tensors "
  + "between devices"
  " may slow down your model");
  case tensorflow::DEVICE_PLACEMENT_WARN:
  LOG(WARNING) << "before computing " << op->name << " input #" << i
  