[hive] Check if destination empty before export data to hdfs

Steps to reproduce:

• Go to Hive query editor
• Run "select * from sample_07"
• Choose from the download options export to HDFS.
• Choose option "All" but not "limit 100000" and enter a path like /user/hive/warehouse/customers
• Check /user/hive/warehouse/customers and see that the directory was purged and replaced with only data from sample_07.

Fix: When a user try to export query result to a non-empty directory, it will give a warning message "the destination is not an empty directory!"