log a warming when warmup ends without detecting steady state

[alexeyr]

Currently it isn't easy to notice in the output when warmup fails to reach steady state: it's just a missing info line. I think it would be good to issue a warning in this case.

[axel22]

739dc58

Since a Warmer is run more than once per benchmark (once per JVM instance), issuing a warning directly from the warmer would emit multiple warning lines when a benchmark is run, so I added a log.verbose.
The questions is how would we decide on a warning if some runs had a successful warmup and others not? Or would you suggest emitting multiple warnings per test?

[alexeyr]

For my own usecase multiple warnings per test would be fine, but I can see it being a problem in other circumstances. Need to think about it some more.

[axel22]

We could do it so that when there's at least one warmup warning, we emit one warning once the test completes.

This would require adding some additional infrastructure to allow the spawned JVM processes to communicate the warmup warning back to the host JVM.