code: private MemcacheContent invalidChunk(Exception cause) {
          state = State.BAD_MESSAGE;
          MemcacheContent chunk = new DefaultLastMemcacheContent(Unpooled.EMPTY_BUFFER);
          chunk.setDecoderResult(DecoderResult.failure(cause));
          return chunk;
      }
docs: Helper method to create a content chunk indicating a invalid decoding result.
  
  @param cause the cause of the decoding failure.
  @return a valid content chunk indicating failure.