Bug 25508: Only return renewal outcomes to the controller

Bug 25508: Only return renewal outcomes to the controller

There are a few cases where the `renew_item` method in
Koha::Account::Line will return `undef`. For these cases, we should not
pass the error up the chain to the controllers as it leads to malformed
error messages in the UI.

Test plan
1 - Make sure FinesMode is on, RenewAccruingItemWhenPaid is off
2 - Checkout an item to a patron and make it overdue (can backdate the
    checkout)
3 - Make sure the itemtype has fines that will be charged
4 - Charge the fines:
    Set finesMode = production
    perl misc/cronjobs/fines.pl -v
5 - Check the fine appears on the patrons account
6 - Pay off the fine
7 - Receive alert after payment that reads: "The fines on
    the following items were paid off, renewal results are
    displayed below:
        No title ( ): Not renewed - Unknown error"
8 - Apply the patch
9 - Repeat steps 1 through 6 and note that you no longer trigger the
    error message.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>