Error handling

[piotr-iohk]

This PR addresses #65 and introduces improved error handling. Errors are now returned in JSON rather then plain text. For instance:

Before:

$ curl -L -X POST 'http://localhost:3000/network/list' -H 'Content-Type: application/json' -H 'Accept: application/json' -d '{}' 
GraphQL query failed

After:

$ curl -L -X POST 'http://localhost:3000/network/list' -H 'Content-Type: application/json' -H 'Accept: application/json' -d '{}' 
{
  "code": 3,
  "description": "The GraphQL query failed.",
  "details": {
    "error": "Error making HTTP request: error sending request for url (http://localhost:3085/graphql)",
    "extra": "Internal POST to Mina Daemon failed"
  },
  "message": "GraphQL query failed: Error making HTTP request: error sending request for url (http://localhost:3085/graphql)",
  "retriable": true
}

The PR also includes a rather comprehensive test suite to verify error properties.