input: Write a function that takes in a positive integer and uses a while loop to output the numbers counting down from that integer to 1. If the input is not a positive integer, return an error message.
  assert countdown_to_one(6) == None
  assert countdown_to_one(3) == None
  assert countdown_to_one(0) == "Error: Input must be a positive integer."
  assert countdown_to_one(-5) == "Error: Input must be a positive integer."
output: def countdown_to_one(n):
      if n <= 0:
          return "Error: Input must be a positive integer."
      while n > 0:
          print(n)
          n -= 1