input: Write a function to find the index of a substring in a given string and print a message if it's not found.
  assert find_substring("The quick brown fox", "fox") == 16
  assert find_substring("Hello World", "World") == 6
  assert find_substring("Python programming", "Java") == "Substring not found!"
output: def find_substring(main_string, sub_string):
      index = main_string.find(sub_string)
      if index == -1:
          return "Substring not found!"
      return index