input: Write a python function to find the starting index of a substring in a given string.
  assert search_substring("hello world", "or") == 7
  assert search_substring("testcase", "case") == 4
  assert search_substring("abcdef", "gh") == -1
output: def search_substring(s, sub):
      try:
          return s.index(sub)
      except ValueError:
          return -1