docs: def _check_users(users):
  
      '''Checks if the input dictionary of users is valid.'''
  
      messg = ''
      valid = True
  
      for user, user_details in six.iteritems(users):
          if not user_details:
              valid = False
              messg += 'Please provide details for username {user}.\n'.format(user=user)
              continue
          if not (isinstance(user_details.get('level'), int) or 0 <= user_details.get('level') <= 15):
              # warn!
              messg += 'Level must be a integer between 0 and 15 for username {user}. Will assume 0.\n'.format(user=user)
  
      return valid, messg