input: Write a function to modify the provided Python code to handle errors. If the specified file is not found, print "The file specified does not exist." If the file is not in a valid JSON format, print "The file specified is not in a valid JSON format." The function should be named `read_data_from_json` and take a single parameter `filename`. Provide assert test cases to demonstrate the functionality.
  
  def read_data_from_json(filename):
      try:
          with open(filename, 'r') as f:
              data = json.load(f)
          return data
      except FileNotFoundError:
          print("The file specified does not exist.")
      except json.decoder.JSONDecodeError:
          print("The file specified is not in a valid JSON format.")
  
  assert read_data_from_json('existing_valid_file.json')  # Should return the JSON data if valid
  assert read_data_from_json('nonexistent_file.json')  # Should print "The file specified does not exist."
  assert read_data_from_json('invalid_json_file.json')  # Should print "The file specified is not in a valid JSON format."
output: def read_data_from_json(filename):
      try:
          with open(filename, 'r') as f:
              data = json.load(f)
          return data
      except FileNotFoundError:
          print("The file specified does not exist.")
      except json.decoder.JSONDecodeError:
          print("The file specified is not in a valid JSON format.")