logstash flush should not die because of enospc

```
Uncaught exception:
  (Sys_error "No space left on device")

Raised by primitive operation at Devkit_core__Logstash.log.object#flush in file "logstash.ml", line 250, characters 44-54
Called from Devkit_core__Logstash.setup_error_log.(fun) in file "logstash.ml", line 263, characters 6-30
Called from Devkit_core__Log.State.Put.output in file "log.ml", line 100, characters 47-66
```