Divelogs.de import empty value fix

This checks that weight and dive durantion are defined before importing.
(Empty value concatenated with unit gives an error on import.)

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Display the filename on error message

Display the filename in error message instead of just text 'ZIP file'

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>