[Serve] Display error message on old service before update mode is introduced.

[cblmemo]

Seems like in the following code, we just silently ignored the mode argument.

skypilot/sky/serve/serve_utils.py

Lines 952 to 966 in d5b6d89

	@classmethod
	def update_service(cls, service_name: str, version: int, mode: str) -> str:
	code = [
	# Backward compatibility for old serve version on the remote
	# machine. The `mode` argument was added in #3249, and if the remote
	# machine has an old SkyPilot version before that, we need to avoid
	# passing the `mode` argument to the job_lib functions.
	# TODO(zhwu): Remove this in 0.7.0 release.
	f'mode_kwargs = {{"mode": {mode!r}}} '
	'if getattr(constants, "SERVE_VERSION", 0) >= 1 else {}',
	f'msg = serve_utils.update_service_encoded({service_name!r}, '
	f'{version}, **mode_kwargs)',
	'print(msg, end="", flush=True)',
	]
	return cls._build(code)

Expected behaviour: if a user specified a rolling update on an old service, we should raise an error says that rolling update is not supported on this service.