Bug 11803: use $dbh consistently in _koha_modify_item

This is just some code cleanup, no behavior change expected.
Also replacing errstr with err in testing the results. (See DBI.)

Test plan:
Modify an item and save it.

Followed test plan. No problems found.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>