404 on DELETE member w/ invalid ID

The fact that we validate the member ID as a hex-encoded uint64 is an optimization in the server. Failing this check does not warrant a different user-facing error code than if that ID was valid but does not represent an existing member.

Rebased on #1492
10.4.1 400 Bad Request

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.

If etcd could tell the user that the id is malformed, why do not tell them? yea, it is an id not found, but more accurately the the format of id is incorrect.