Per discussion, this field isn't really specified in a way that lets
us fill it sensibly at the moment.  So for now, we're going to just
omit it.

Additionally, we said that we'd Report on our errors; this branch
changes the implementation of RpcError to do that.

Question: Will the blanket implementation for Into<RpcError> make
it harder to re-add a Data field later on if we want to do so?