Make CLI convert_to_parquet not raise error if no rights to create script branch
Make CLI convert_to_parquet not raise error if no rights to create "script" branch.

Not that before this PR, the error was not critical because it was raised at the end of the script, once all the rest of the steps were already performed.

Fix #6901.

Bug introduced in datasets-2.19.0 by:
- #6809