Write a unique_characters() function that determines if any given string has all unique characters (i.e. no character in the string is duplicated). If the string has all unique characters, the function should return True. If the string does not have all unique characters, return False.

For example, unique_characters("apple") should return False.

This challenge and variations of it were reported to have been asked at interviews with Google. If you’ve covered the material in Pass the Technical Interview with Python or an equivalent, you should be able to solve this challenge. If you have trouble, try refreshing your knowledge there first.

Note that if the function is called with an empty string (“”), the program should return an error message.