Why a sidecar command did not answer with what was asked for. **Two kinds, because only two things can be done about one.** A `Fault` is the sidecar breaking - an exception escaped a handler, so an invariant the simulator maintains no longer holds. That is a bug in Issie: it is logged as an error, which in a debug build puts it and its stack on the screen. A `Refusal` is the sidecar declining something it anticipated and wrote a message for - a stale epoch, a cycle it has not run to, a sheet that did not decode - which the caller is expected to handle, and which is a warning unless the caller cannot. **Anything unclassified is a Fault.** A failure nobody has said is expected is a bug until somebody says otherwise; the alternative default quietly downgrades whatever is forgotten. The strings are Protocol.FaultKind and Protocol.RefusalKind, mirrored here because the renderer cannot see that file - the sidecar references the renderer, not the other way round.