--index_on_timedelta FLAG added to convert_init_to_valid_time.py.

```
INDEX_ON_TIMEDELTA = flags.DEFINE_boolean(
    'index_on_timedelta',
    True,
    help=(
        'Whether to index the output on the timedelta (and drop initial time).'
        ' If False, index on the initial time axis (and drop the timedelta).'
    ),
)
```
PiperOrigin-RevId: 686675196

xarray_beam/_src/zarr.py

@@ -310,7 +310,7 @@ def validate_zarr_chunk(
       if not expected_index.equals(actual_index):
         raise ValueError(
             f'template and chunk indexes do not match for dim {dim!r}:\n'
-            f'{expected_index}\nvs.\n{actual_index}'
+            f'{expected_index}\nvs.\n{actual_index}, and {key}.'
         )
 
   if zarr_chunks is None: