Improve error message when terminating due to a predicate

[mum4k]

Nighthawk will sometimes terminate some of its worker threads early, before the specified --duration has elapsed. It reports the error:

[E] Exiting due to failing termination predicate

This error message should tell the user which termination predicate failed, that is, which counter was exceeded, and what its final value was.

This would be useful debugging information because the user may have specified multiple termination predicates.

[jmarantz]

@mergeconflict