Bug 19420: Improve display of errors from failure of uploading file during stage...

Bug 19420: Improve display of errors from failure of uploading file during stage import

To test:
1 - sudo chown www-data:www-data /tmp/koha_kohadev_upload/ (or your
                relevant dir and user)
2 - Attempt to stage a file
3 - Error message is: Upload status: Failed
4 - Apply patch
5 - Repeat upload
6 - Error is now: Failed - Directory is not writeable:
/tmp/koha_kohadev_upload
7 - Trigger the other errors if you can and note better responses

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>