SI-7999 s.u.c.NonFatal: StackOverflowError is fatal

As demonstrated in
https://groups.google.com/d/topic/scala-language/eC9dqTTBYHg, SOEs
should be considered fatal, because all popular JVM implementations
seem to run into inconsistent state (ignoring finally blocks leading
to not running monitorExit, leading to locks not being unlocked, ...)
if one just pushes them enough.