Avoid raising an exception when the user to be added to an LDAP group is already a member of it.
requested to merge birin.sanchez/ldh_middleware:346_add_existing_user_to_LDAP_group_crash into master
This commit prevents raising an exception when adding user to an LDAP group where the user is already member of it. This can cause having customers with some services enabled and other not even when they should be enabled.
I have tested this in my test environment and seems to be working fine.