nixos/systemd-boot: refactor json.load() logic for better error message (#421916)

def get_bootspec(profile: str | None, generation: int) -> BootSpec:

    system_directory = system_dir(profile, generation, None)

    boot_json_path = os.path.realpath("%s/%s" % (system_directory, "boot.json"))

    boot_json_path = os.path.join(system_directory, "boot.json")

    if os.path.isfile(boot_json_path):

        boot_json_f = open(boot_json_path, 'r')

        with open(boot_json_path, 'r') as boot_json_f:

            # check if json is well-formed, else throw error with filepath

            try:

                bootspec_json = json.load(boot_json_f)

            except ValueError as e:

                print(f"error: Malformed Json: {e}, in {boot_json_path}", file=sys.stderr)

                sys.exit(1)

    else:

        boot_json_str = run(

            [