Handle Terminated to avoid endless DeathPactExceptions.

Credit to Roland Kuhn, Akka's tech lead, for pointing out this
various obvious fix, but StandaloneExecutorBackend.preStart's
catch block would never (ever) get hit, because all of the
operation's in preStart are async.

So, the System.exit in the catch block was skipped, and instead
Akka was sending Terminated messages which, since we didn't
handle, it turned into DeathPactException, which started
a postRestart/preStart infinite loop.