Better error codes and message when sebak has problems

[outsideris]

When sebak has some problems like the consensus is failed or doesn't work, it should return HTTP 500 status code and properly message. For example, when sebak can not make consensus and can not make new block, it still return 400 error to client like sebak is ok. I also think it is hard to detect consensus problems but it is helpful for clients to show error messages properly.

[kfangw]

#759 may solve this issue. But If I have a chance, I dig into it.