Attempting to remove nonexistent member mishandled

1. Spin up a single-node etcd cluster.
2. Attempt to remove a non-existent member:

```
% curl -i -X DELETE http://127.0.0.1:4001/v2/admin/members/2745e2525fce8fe
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
X-Etcd-Cluster-Id: a940b9ecc6609d71
Date: Wed, 29 Oct 2014 21:30:59 GMT
Transfer-Encoding: chunked

Internal Server Error
```

There are two problems:
1. A 500 is returned rather than a 404.
2. etcd is logging stuff, but it really doesn't need to log anything since this is a user-level error:

```
2014/10/29 14:30:59 etcdhttp: remove node 2745e2525fce8fe
2014/10/29 14:30:59 etcdhttp: error removing node 2745e2525fce8fe: etcdserver: ID not found
2014/10/29 14:30:59 etcdserver: ID not found
```
@jonboulle should we log this? i think this should be logged when etcdserver processing the remove request.