Ref T12450. Currently, if a write fails, we stop and don't try to write to other index services. There's no technical reason not to keep trying writes, it makes some testing easier, and it would improve behavior in a scenario where engines are configured as "primary" and "backup" and the primary service is having some issues.
Also, make "no writable services are configured" acceptable, rather than an error. This state is probably goofy but if we want to detect it I think it should probably be a config-validation issue, not a write-time check. I also think it's not totally unreasonable to want to just turn off all writes for a while (maybe to reduce load while you're doing a background update).