Error responses

[metachris]

Issue for discussing various parts of the error responses.

• There was some discussion about sending the SignedBlindedBeaconBlock in SSZ format, using a Content-Type: application/octet-stream request header. If mev-boost / the relay does not accept that format, should it return status code 415? https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/415
• Clarify errors on registrations #8

[ralexstokes]

I think we should extend the builder specs to mirror the beacon API specs here in terms of offering a SSZ binary route. 415 sounds like a great status code in the event the builder software does not support the codec.