Bug 33608: Polish Koha::Statistic further

Bug 33608: Polish Koha::Statistic further

Adding exceptions, removing croaks.
No exception in new for unknown hash keys, store will catch that.
Prepare switching amount parameter to value (db column name).

Test plan:
Run t/db_dependent/Stats.t
Run t/db_dependent/Koha/Items.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
[EDIT] Fixed the mandatory check in Stats.t. Removed key_or_default.
       Additional tidy.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>