@Override
    public void printStackTrace(PrintStream ps) {
        super.printStackTrace(ps);
        if (nestedThrowable != null) {
            nestedThrowable.printStackTrace(ps);
        }
    }
Exception used for representing that the nickname used by the user is not acceptable. The reason
for this could be that the user has previously registered with the room and the room is
configured so that registered users must join using the reserved nickname. A 406 error code is 
returned to the user that requested the invalid operation.

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