input: Write a function to modify the given Python function `read_data_from_json` to handle errors. Specifically, it should catch and handle `FileNotFoundError` for non-existent files and `json.decoder.JSONDecodeError` for invalid JSON format files.
  
  
  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("nonexistent.json")=="The file specified does not exist."
  assert read_data_from_json("invalid_json.json")=="The file specified is not in a valid JSON format."
  assert read_data_from_json("valid_json.json")==expected_json_data
  assert read_data_from_json("empty.json")=={}
  assert read_data_from_json("empty_string.json")=="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:
          return "The file specified does not exist."
      except json.decoder.JSONDecodeError:
          return "The file specified is not in a valid JSON format."