Rollup node: don't overwrite already populated data dir on import
Context
Importing a snapshot in the data directory or a rollup node which is already populated will now fail by default. One can use the --force option to still import and overwrite the non-local data of the rollup node (sanity checks are still run).
The new default behavior aligns with the one for snapshot import of the Octez node. (Thanks to @chrispinnock for feedback.)