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
The alternative here is to elevate this requirement into the spec. So far, we've just said that a Member has a string "ID" field that should be used when DELETE'ing that Member.