Bug 20590: Include Koha::Exceptions::Exception

Bug 20590: Include Koha::Exceptions::Exception

This patch makes Koha::Exceptions::Object include
Koha::Exceptions::Exception so it is not required to include it
excplicitly for using this exceptions.

To test:
- Run:
  $ kshell
 $k prove t/Koha/Exceptions.t
=> FAIL: Tests fail because Koha::Exceptions::Exception is not
(implicitly or explicitly included)
- Apply this patch
- Run
 k$ prove t/Koha/Exceptions.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>