Status code when attempting to create an identity that already exists

When starting a FireFly stack using a pre-existing Fabric network, the org name was already registered (from a previous failed run of the FireFly CLI). Fabconnect returns an HTTP `500` which causes the CLI to exit.

```Error: http://127.0.0.1:5102/identities [500] {"error":"failed to register user: failed to register user: Response from server: Error Code: 74 - Identity 'org_0' is already registered\n"}```

Because this is not the fault of Fabconnect, and repeatedly sending this request will not have a different result, I do not think `500` is the correct status code here. Perhaps `409`? Open to input.