input: def read_data_from_json(filename):
      """Read data from a JSON file and handle potential errors such as file not found or invalid JSON format.
      >>> read_data_from_json('existing_file.json')
      {data from the JSON file}
      >>> read_data_from_json('non_existent_file.json')
      The file specified does not exist.
      >>> read_data_from_json('invalid_json_file.json')
      The file specified is not in a valid JSON format.
      """
output:     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."