Document 412 status code in "Response Codes" section

[3flex]

This appears to be used only when attempting to enable vanity codes but they're not enabled for the current plan. Code's not mentioned in https://developer.dnsimple.com/v2/#response-codes.

Something confusing related to this is that enabling vanity servers when plan doesn't support it returns 412 with this response:

{
  "message": "Feature not enabled: vanity_name_servers"
}

But attempting to set regions on a record when plan doesn't support it returns 400 with this response:

{
  "message": "Regions are not allowed for your plan.",
  "errors": {}
}

The inconsistent error codes make it difficult to handle a "feature not enabled" response from the server. Sorry I know this isn't documentation related but not sure where it's best to bring it up. Having a different code from 400 (which I read as being invalid JSON input in the request body) would be useful.