Adding a duplicate Group Membership Status throws an error

[jefft]

In Jethro, every group member has a membership 'status', from a list of available Group Membership Status Options configured under Admin -> System Configuration.

If a value is accidentally duplicated, as 'None' is in this example:

then on saving, Jethro blows up with an error:

PHP message: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'None' for key 'label' - Line 383 of /srv/www/jethro/2.36.0/app/views/view_10_admin__6_system_configuration.class.php

By contrast, duplicating Person Status results in a decent error message:

[radarhere]

I've created #1175