Gradle jmh task still succeed when JMH fails.

[robin-aws]

Describe the bug
See title.

To Reproduce
Create a benchmark method that always throws an exception and ensure failOnError is true. The exception will print out and JMH will report failure, but Gradle will still report "BUILD SUCCESSFUL". (Working on setting up this repo locally and making a unit test to confirm).

Root cause is failing to check the exit code after spawning the process to call JMH: https://github.com/melix/jmh-gradle-plugin/blob/master/src/main/java/me/champeau/jmh/JMHTask.java#L70