input: Write a function to modify the given Python code to handle errors. The function should open and read a JSON file, but it should also handle cases where the file does not exist or if the file is not a valid JSON. If the file does not exist, it should print "The file specified does not exist." If the file is not a valid JSON, it should print "The file specified is not in a valid JSON format."
  
  assert read_data_from_json("test.json")==None
  assert read_data_from_json("nonexistent_file.json")=="The file specified does not exist."
  assert read_data_from_json("invalid_json_file.json")=="The file specified is not in a valid JSON format."
output: def read_data_from_json(filename):
      try:
          with open(filename, 'r') as file:
              data = json.load(file)
              return data
      except FileNotFoundError:
          return "The file specified does not exist."
      except json.JSONDecodeError:
          return "The file specified is not in a valid JSON format."