input: 
  def read_data_from_json(filename):
      """
      Read data from a JSON file and handle errors related to file not found or invalid JSON format.
      
      >>> read_data_from_json('valid_test_file.json')
      {'key': 'value'}
      >>> read_data_from_json('non_existent_file.json')
      The file specified does not exist.
      >>> read_data_from_json('invalid_format.json')
      The file specified is not in a valid JSON format.
      """
      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.")
  
output:     except FileNotFoundError:
          return "The file specified does not exist."
      except json.decoder.JSONDecodeError:
          return "The file specified is not in a valid JSON format."