R1:304d19f92a7b

After a fulltext write to a particular service fails, keep trying writes to other services

Summary:
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.