RPC error code

[sgerbino]

Resolves #667.

Brief description

Add code to RPC response JSON data.

Checklist

• I have built this pull request locally
• I have ran the unit tests locally
• I have manually tested this pull request
• I have reviewed my pull request
• I have added any relevant tests

Demonstration

❯ curl -d '{"jsonrpc":"2.0", "method":"chain.read_contract", "params":{"args":"","contract_id":"z11111111111111111112","entry_point":1249216561}, "id":1}' http://localhost:8080/
{"jsonrpc":"2.0","error":{"code":-32603,"message":"contract does not exist","data":"{\"code\":102}"},"id":1}

...
    invalid_contract             =  102;
...

[mvandeberg]

Do we want the code to be string json or inline embedded?

"data":"{\"code\":102}"} vs "data":{"code":102}}

[sgerbino]

The problem is defining the inline embedded strategy in protobuf. This is a koinos.rpc.error_response which ultimately may or may not be JSON.

Defined here:
https://github.com/koinos/koinos-proto/blob/master/koinos/rpc/rpc.proto

We might be able to define something like this:

message data_record {
   string key = 1;
   google.protobuf.Any value = 2;
}

Then redefine error_response like this:

message error_response {
   string message = 1;
   repeated data_record data = 2;
}