input: Write a function to generate a Python program that adds two numbers and then doubles the result. The function should take two numbers as input and return the final result. If either of the numbers is missing, the function should return an error message.
  assert double_add(3, 4)==14
  assert double_add(-2, 2)==0
  assert double_add(10, -10)==0
  assert double_add(7, None)=='Error: Missing input'
  assert double_add(None, 5)=='Error: Missing input'
output: def double_add(a, b):
      if a is None or b is None:
          return 'Error: Missing input'
      return (a + b) * 2