Fixes for the admin meta-contexs features

Squashed commit of the following:

commit af1cf5e27d876eb1704bb017102ec387576be618
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Tue Jun 3 09:28:47 2014 +0200

    Squashed commits:

    fix removing meta context from a meta context group

    proper action labels, rescue action exceptions

    fix deleting a context when it is associated with media sets