input: Write a function to create a Python program that takes in a positive integer, then uses a while loop to output the numbers counting down to 1. If the input is not a positive integer, return an error message.
  assert countdown(6) == "6\n5\n4\n3\n2\n1\nDone!"
  assert countdown(3) == "3\n2\n1\nDone!"
  assert countdown(-1) == "Error: Please enter a positive integer."
  assert countdown(0) == "Error: Please enter a positive integer."
output: def countdown(n):
      if n <= 0:
          return "Error: Please enter a positive integer."
      result = ""
      while n > 0:
          result += f"{n}\n"
          n -= 1
      result += "Done!"
      return result