Async API operations don't return error codes

Synchronous snapd API operations return an error string and optional "kind" on error:

{
    "type": "error"
    "status-code": 401,
    "status": "Unauthorized",
    "result": {
        "message": "access denied",
        "kind": "login-required",
    }
}

Async operations can fail (as returned from /v2/changes):

{
    ...
    "status": "Error",
    "err": "Error text"
}

There is no error kind reported here, so a client can't tell what caused the async operation to fail. Suggest solution is to add an "errKind" field or similar to this object.