input: Write a python function to handle custom exceptions when performing integer division.
  assert divide(10, 2) == 5
  assert divide(9, 3) == 3
  try:
      divide(5, 0)
  except CustomException as e:
      assert str(e) == "Division by zero is not allowed"
output: class CustomException(Exception):
      pass
  
  def divide(a, b):
      if b == 0:
          raise CustomException("Division by zero is not allowed")
      return a // b