Noticed that ACL endpoints return 500 status code for user errors.  This
is confusing and can lead to false monitoring alerts.

Here, I introduce a concept of RPCCoded errors to be returned by RPC
that signal a code in addition to error message.  Codes for now match
HTTP codes to ease reasoning.

```
$ nomad acl bootstrap
Error bootstrapping: Unexpected response code: 500 (ACL bootstrap already done (reset index: 9))

$ nomad acl bootstrap
Error bootstrapping: Unexpected response code: 400 (ACL bootstrap already done (reset index: 9))
```