Missing group 500 error.

This fixes an issue where a 500 error occurs if the use attempts to access a group that doesn't exist. This creates makes the system behave the same as if the user didn't have access to the group.

The current functionality returns a 404 if the user doesn't have access and 500 if the group does not exist.

This functionality returns 404 if the user doesn't have access or the group doesn't exist.