improve error message if parquet target exists

The current error message does not tell you which parquet directory is not empty. If the non-empty path would be shown, the error message would be more helpful.

https://github.com/GIScience/ohsome-planet/blob/319c229cd83a49cbebc8567aacbdc63bac6769b8/ohsome-planet-cli/src/main/java/org/heigit/ohsome/planet/cmd/Contributions.java#L135

Read ohsome-planet-cli/src/main/java/org/heigit/ohsome/planet/cmd/Contributions.java around line 135 and trace the check for an existing parquet target. Update the error wording so it identifies the non-empty parquet directory, then verify the message when the target already contains files.

Written by the indexing model from the issue text.