Better handling for when bundle config is not set for group type/

[rosiel]

When you install this module there's a mandatory config that you have to set up at admin/config/access-control/islandora_group AFTER setting up a (every) group type. Otherwise you get php's white screen of death (WSOD).

1 - this should be documented in the Islandora Group Documentation. One must do this before creating an actual group, because you will get a WSOD.

2 - We should handle the case where the config is not set and issue a validation error directing the user to go to the config page, instead of a WSOD.