input: Write a function to check if a string is valid according to given rules, ensuring it contains only lowercase characters and digits.
  assert check_string_validity("betterthanever123", "String should contain only lower case characters and digits") == True
  assert check_string_validity("Betterthanever", "String should contain only lower case characters and digits") == False
  assert check_string_validity("valid123", "String should contain only lower case characters and digits") == True
  assert check_string_validity("Valid!@#", "String should contain only lower case characters and digits") == False
output: def check_string_validity(s, error_message):
      return s.islower() and s.isalnum()