Fix a bug where groups were disclosed to unauthorized users.

Users without the permission to view a group were given a 404 if the
group did not exist, but a 403 if it existed but they lacked
permission. This change includes a quick fix to return a 404 if the
user does not have the ability to view a group.

Signed-off-by: Tyler O'Meara <Tyler@TylerOMeara.com>
Merge pull request #339 from Acidity/feature/groupsFixInfoDisc

Fix a bug where groups were disclosed to unauthorized users.