Preparatory steps for the new consumer

Some miscellaneous changes I found necessary while developing it, that have been
separated out so they can land first:
- define `type none struct{}` for convenience
- implement `FetchResponse.AddError()`
- prefer the seedBroker when fetching metadata, it makes client behaviour much
  more deterministic and easier to test in multi-broker setups

@wvanbergen @graemej @mkobetic @marc-barry