input: Write a function to take a positive integer and use a while loop to output the numbers counting down from the given integer to 1, followed by the word "Done!". If the given integer is not positive, return an error message.
  assert countdown(6) == "6\n5\n4\n3\n2\n1\nDone!"
  assert countdown(-3) == "Error: Input must be a positive integer."
  assert countdown(1) == "1\nDone!"
  assert countdown(0) == "Error: Input must be a positive integer."
  assert countdown(5) == "5\n4\n3\n2\n1\nDone!"
output: def countdown(n):
      if n <= 0:
          return "Error: Input must be a positive integer."
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!"
      return result