Challenge 8 - Unique characters in a string
Write a 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, `uniqueCharacters('apple')` should return `false`.
Note that if the function is called with an empty string, it should return an error message.
Input:
Output: undefined