Bug 21903: (follow-up) koha-dump can export uploaded and temporary uploaded files

Bug 21903: (follow-up) koha-dump can export uploaded and temporary uploaded files

Minor improvment on display.
Solve the error message if the folder for uploaded temp files
doesn't exist.

Sponsored-by: Orex
Test plan:
- Apply the patch
- Try using koha-dump without any option
- Try using koha-dump with --uploaded_files
- Try using koha-dump with --uploaded_temp_files
- Try using koha-dump with both of the options above

Expected results: the .tar.gz dump will include uploaded and/or temporary
uploaded files if requested.
Change message and solve the error message if no temporary folder

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>