input: def read_data_from_json(filename):
      """
      Read data from a JSON file and handle potential errors.
      >>> read_data_from_json('existing_file.json') # Assuming 'existing_file.json' exists and is valid JSON
      {'key': 'value'}
      >>> read_data_from_json('non_existent_file.json') # Assuming 'non_existent_file.json' does not exist
      "The file specified does not exist."
      >>> read_data_from_json('invalid_json_file.json') # Assuming 'invalid_json_file.json' is not valid 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."