[fix] fixed wrong equals comparison (UUID with String)
[enhancement] introduced OsiamException as base class for all exceptions
[cleanup] changed comment to javadoc
[fix] using ResourceDao.getById() instead of try-catch block
[fix] not using MultiValuedAttribute but MemberRef now
[enhancement] added group related exception handling
[enhancement] added user related exception handling
[enhancement] improved exception handling
[cleanup] removed some TODOs that do not have to be done
[fix] fixed ConcurrentModificationException when removing all members
[fix] fixed test to reflect updated GroupConverter