Bug 10150 - koha-email-disable error handling

koha-email-disable now
- Checks the instance exists.
- Checks if email is already disabled.

Regards
To+

Sponsored-by: Universidad Nacional de Córdoba
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>