improve error message if parquet target exists

[joker234]

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.

ohsome-planet/ohsome-planet-cli/src/main/java/org/heigit/ohsome/planet/cmd/Contributions.java

Line 135 in 319c229

System.out.println("parquet-data directory is not empty!");