`dond` error in finally block

`finally` block in `dond` ([here](https://github.com/QCoDeS/Qcodes/blob/master/qcodes/dataset/dond/do_nd.py#L738)) relies on a parameter from try block.
This results in an `UnboundLocalError` when the exception happens before `datasavers` is assigned.

```python
try:
with _catch_interrupts() as interrupted, ExitStack() as stack, params_meas_caller as call_params_meas:
datasavers = [
stack.enter_context(group.measurement_cxt.run())
for group in measurements.groups
]
...
finally:
for datasaver in datasavers:
...
```

Start in qcodes/dataset/dond/do_nd.py at the finally block around line 738, and trace how datasavers is assigned inside the try block. Reproduce an exception before that assignment and confirm the fix prevents UnboundLocalError while preserving the intended finally-block cleanup behavior.

Written by the indexing model from the issue text.