Clarify how `RTLM6` and `RTLC6` should handle unexpected, missing or incompatible input

[VeskeR]

See #343 (comment) comment and ably-js implementation https://github.com/ably/ably-js/blob/af2ce0eef23068b19cccd2677453386cc470372d/src/plugins/objects/livemap.ts#L443-L489

[sacOO7]

• In case of kotlin, we catch all types of exceptions at one place where processing of incoming liveobject message starts.
  https://github.com/ably/ably-java/blob/993b308f93090c4d6ccda4870ac21cf64f41f7b9/liveobjects/src/main/kotlin/io/ably/lib/objects/DefaultRealtimeObjects.kt#L231-L244
• I believe we add an explicit liveobjects spec point for the same and implement for robustness across SDKs.