I need this RPC to write tests and investigate test failures. It will probably be useful as well to help users troubleshoot proofs that they generated.
To return a proper JSONRPCError in case of invalid hexadecimal input, I had to implement a `bool DecodeHexProof(Proof &proof, const std::string &strHexProof)` based on a similar method used for decoding tx by `decoderawtransaction`. This can be used for other RPCs that take proofs as input.