Bug 32334: (follow-up) Improvement for schema parameter

Bug 32334: (follow-up) Improvement for schema parameter

Essentially only adds a warn, some cosmetic changes too.

Test plan:
Copy your kohastructure to xx.sql.
Run sync_db_comments.pl -schema xx.sql. You will see usage.
Run sync_db_comments.pl -schema xyz.sql. You will see a warn and
the usage statement.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>