Clarify errors on registrations

The validator registration endpoint currently takes a single registration with the understanding that relay muxers will send the registration to each relay they are aware of.

The spec for this endpoint returns a binary success/error response which precludes the case where a some relays succeed where others fail when processing a registration.

I think we want to erase this level of detail from the API in which case we don't need to change anything. For example, a caller of this endpoint who receives an error should assume the registration has completely failed and they need to try again (e.g. beacon node queues a retry for later). If they get a success, they should assume the responsibility on promulgating their registration is now with the muxer software (and the implication is that internally, the muxer may retry against transient relay failures, etc.)

If this is *not* the intended semantics, then we should extend the response so many errors can be returned.