Bug 17494: (QA followup) Fix exception name

Bug 17494: (QA followup) Fix exception name

This patch changes the exception name to match the package name. And
also adds a test for the thrown exception message.

Also creates a general Koha::Exception::Patron::Modification exception
to make the new one inherit from it, following the common use in the codebase.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Mason James <mtj@kohaaloha.com>
(cherry picked from commit 9a301836714924581b806cd7c728695625ddf3cc)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>