Bug 18174: Add update to Koha::Object

Bug 18174: Add update to Koha::Object

Since Koha::Objects has a update method, we should allow it too in
Koha::Object. Note that it is just redirecting to DBIx immediately.

Changed the exception when the method generates an error. The previous
code suggested that the method was not found, but this is not the case.

Test plan:
Run t/db_dependent/Koha/Object.t

Followed test plan, tests passes OK.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>