public TApplicationException exception() {
        if (!isException()) {
            throw new IllegalStateException("not an exception but a reply");
        }
        assert exception != null;
        return exception;
    }
Returns the exception of this reply.

@throws IllegalStateException if the type of this reply is not {@link TMessageType#EXCEPTION}
thrift/thrift0.13/src/main/java/com/linecorp/armeria/common/thrift/ThriftReply.java