cormoran

Implemented `completeExceptionally` in `CompletionImpl`. I made it package private to keep it internal use only. BTW, I think propagate and logging un-caught exception thrown by async processor is nicer, since...

Sorry for long suspending. Added integration test like existing metrics test, since metrics are globally defined and it's hard to test with smaller scope.

I also encountered this problem with restartable job. (`createJobExecution` could be called from different process before starting the execution.) This behavior seems introduced from https://github.com/spring-projects/spring-batch/pull/659. Adding the guard condition `execution.isStarting()`...

https://github.com/spring-projects/spring-batch/pull/4113 also fixes the same issue with more fundamental way, although my PR might have fewer change to interface. Feel free to close it if authors will accept https://github.com/spring-projects/spring-batch/pull/4113.

> By the way, in which case are you throwing an exception from the mapper? @minwoox In our case, json parser throws exception if the json format is invalid like...