Make vnode terminate backend for any exit reason

This should fix issue basho/riak_test#137, where partition repair was
killing vnodes with reason kill_for_test and experiencing sporading
bitcask data corruption.
Wrapping pool shutdown in try/catch

Making sure that we really always call terminate to avoid other issues
like basho/riak_test#137
Add stacktrace to terminate error catch log
Merge pull request #264 from basho/eas-fix-vnode-terminate-abnormal-reason

Make vnode terminate backend for any exit reason