Give a nicer error message back if a user runs subdomain command with their existing subdomain

[kristianfreeman]

I was expecting that I would need to run the new subdomain command with my workers.dev subdomain, to register it in the configuration. I did that and got this error back:

worker $ wrangler subdomain signalnerve
🐌  Registering your subdomain, signalnerve.workers.dev, this could take up to a minute.
Error: ErrorMessage { msg: "⛔ This account already has a registered subdomain. You can only register one subdomainper account. Your subdomain is signalnerve.workers.dev \n Status Code: 409 Conflict\n Msg: {\n  \"result\": null,\n  \"success\": false,\n  \"errors\": [\n    {\n      \"code\": 10036,\n      \"message\": \"workers.api.error.account_has_subdomain\"\n    }\n  ],\n  \"messages\": []\n}\n" }

It would be neat to validate the user's existing subdomain against the argument they passed in for the command: if they're the same, we can return something back like "Your domain is already registered!"