input: Write a function to count down from a given positive integer to 1 using a while loop and print each number. If the input is not a positive integer, return an error message. 
  def count_down(num):
      if num <= 0 or not isinstance(num, int):
          return "Error: Input must be a positive integer."
      while num > 0:
          print(num)
          num -= 1
      return "Done!"
  assert count_down(6) == "Done!"
  assert count_down(0) == "Error: Input must be a positive integer."
  assert count_down(-5) == "Error: Input must be a positive integer."
  assert count_down(10) == "Done!"
  assert count_down(1) == "Done!"
output: def count_down(num):
      if num <= 0 or not isinstance(num, int):
          return "Error: Input must be a positive integer."
      while num > 0:
          print(num)
          num -= 1
      return "Done!"