@Override
    public void printStackTrace(PrintStream ps) {
        super.printStackTrace(ps);
        if (nestedThrowable != null) {
            nestedThrowable.printStackTrace(ps);
        }
    }
Exception used for representing that the user needs to register with the room in order to perform 
the requested operation. There are many reasons why a registration-required error could occur 
such as: a user which is not a member tries to join a members-only room. A 407 error code is 
returned to the user that requested the invalid operation.

@author Gaston Dombiak
xmppserver/src/main/java/org/jivesoftware/openfire/muc/RegistrationRequiredException.java