Requirements when response is error

[rweigel]

We need to clarify in the documentation that if there is an error, the only required elements are HAPI and status. For example, the following should be valid

{
    "HAPI": "2.1",
    "status": {
        "code": 1406,
        "message": "HAPI error 1406: unknown id"
    }
}

For < 3.0, this was tested for against a subschema that was not a part of the info schema.