Add test to check how errors are rendered

[leighmcculloch]

What

Add test to check how errors are rendered.

Why

I wrote the test to confirm the behavior.

In output of stellar-xdr I saw the following:

{
  "error": {
    "type_": "contract",
    "code": "existing_value"
  }
}

Writing this test confirmed the correct rendering is:

{
  "error": {
    "contract": 4
   }
}

Committing this test because, why not.