text: Title: tl.files.load_and_assign_npz() does not return 'False'
  Body: ### New Issue Checklist
  
  - [x] I have read the [Contribution Guidelines](https://github.com/tensorlayer/tensorlayer/blob/master/CONTRIBUTING.md)
  - [x] I searched for [existing GitHub issues](https://github.com/tensorlayer/tensorlayer/issues)
  
  ### Issue Description
  
  tl.files.load_and_assign_npz() does not return 'False' when the model is not exist.
  I tried to add 'return False' to the code and it works well.
  tensorlayer/files/utils.py
  
  ```python
  def load_and_assign_npz(sess=None, name=None, network=None):
  
  
      if not os.path.exists(name):
          logging.error("file {} doesn't exist.".format(name))
          return False
  ```
  
  ### Reproducible Code
  
  - Which OS are you using ?
   Ubuntu 16.04 LTS
  
  ```python
      if tl.files.load_and_assign_npz(sess=sess, name=checkpoint_path + 'g_{}.npz'.format(tl.global_flag['mode']), network=net_g) is False:
          tl.files.load_and_assign_npz(sess=sess, name=checkpoint_path + 'g_{}_init.npz'.format(tl.global_flag['mode']), network=net_g)
  ```