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
This is the premise of the PR. The fact that it is malformed does not change the fact that an item with the given identifier does not exist. We're simply optimizing our lookup by verifying the format of the ID.